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dark matter
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{{Hatnote|Not to be confused with antimatter, dark energy, dark fluid, or dark flow. For other uses, see Dark Matter (disambiguation).}}{{Use dmy dates|date=November 2016}}{{Physical cosmology|comp/struct}}{{Beyond the Standard Model|expanded=Evidence}}Dark matter is a theorized form of matter that is believed to account for approximately 80% of the matter in the universe, and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.A small portion of dark matter could be baryonic. See Baryonic dark matter. Dark matter has not been directly observed, but its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained unless more matter is present than can be seen. For this reason, most experts believe dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. The name dark matter refers to the fact that it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible (or 'dark') to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.WEB,weblink Dark Matter, CERN Physics, 20 January 2012, The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.NEWS, Siegfried, T., 5 July 1999, Hidden Space Dimensions May Permit Parallel Universes, Explain Cosmic Mysteries,weblink The Dallas Morning News, Other lines of evidence include observations in gravitational lensing,JOURNAL, Trimble, V., 1987, Existence and nature of dark matter in the universe, Annual Review of Astronomy and Astrophysics, 25, 425–472, 1987ARA&A..25..425T, 10.1146/annurev.aa.25.090187.002233, from the cosmic microwave background, from astronomical observations of the observable universe's current structure, from the formation and evolution of galaxies, from mass location during galactic collisionsweblink and from the motion of galaxies within galaxy clusters. In the standard Lambda-CDM model of cosmology, the total mass–energy of the universe contains 4.9% ordinary matter and energy, 26.8% dark matter and 68.3% of an unknown form of energy known as dark energy.WEB,weblink Planck Mission Brings Universe Into Sharp Focus, NASA Mission Pages, 21 March 2013, WEB,weblink Dark Energy, Dark Matter, NASA Science: Astrophysics, 5 June 2015, JOURNAL, P. A. R., Ade, N., Aghanim, C., Armitage-Caplan, (Planck Collaboration), Planck 2013 results. I. Overview of products and scientific results â€“ Table 9, Astronomy and Astrophysics, 1303, 5062,weblink 22 March 2013, 1303.5062, 2014A&A...571A...1P, 10.1051/0004-6361/201321529, etal, WEB, First Planck results: the Universe is still weird and interesting,weblink Francis, Matthew, 22 March 2013, Arstechnica, Thus, dark matter constitutes 84.5%Since dark energy, by convention, does not count as matter, this is 26.8/(4.9 + 26.8)=0.845 of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content.WEB,weblink Planck captures portrait of the young Universe, revealing earliest light, 21 March 2013, University of Cambridge, 21 March 2013, Sean Carroll, Ph.D., Caltech, 2007, The Teaching Company, Dark Matter, Dark Energy: The Dark Side of the Universe, Guidebook Part 2 page 46, Accessed 7 October 2013, "...dark matter: An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe... it's a different kind of particle... something not yet observed in the laboratory..."WEB, Dark Matter,weblink 10 June 2015, Timothy, Ferris, JOURNAL, Jarosik, N., etal, 2011, Seven-Year Wilson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results, Astrophysical Journal Supplement, 192, 2, 14, 1001.4744, 2011ApJS..192...14J, 10.1088/0067-0049/192/2/14, Because dark matter has not yet been observed directly, it must barely interact with ordinary baryonic matter and radiation. The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular, weakly-interacting massive particles (WIMPs), or gravitationally-interacting massive particles (GIMPs). Many experiments to directly detect and study dark matter particles are being actively undertaken, but none have yet succeeded. Dark matter is classified as cold, warm, or hot according to its velocity (more precisely, its free streaming length). Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists,JOURNAL, Kroupa, P., etal, 2010, Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation, Astronomy and Astrophysics, 523, 32–54, 1006.1647, 2010A&A...523A..32K, 10.1051/0004-6361/201014892, intrigued by certain observations that do not fit the dark matter theory,NEWS, Cooper, Keith, Keith Cooper, Correlation between galaxy rotation and visible matter puzzles astronomers,weblink argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, or entropic gravity. These models attempt to account for all observations without invoking supplemental non-baryonic matter.JOURNAL, Angus, G., 2013, Cosmological simulations in MOND: the cluster scale halo mass function with light sterile neutrinos, Monthly Notices of the Royal Astronomical Society, 436, 202–211, 2013MNRAS.436..202A, 10.1093/mnras/stt1564, 1309.6094, {{toclimit|3}}

History

{{refimprove science | section|date=December 2016}}

Early history

The hypothesis of dark matter has an elaborate history.JOURNAL, de Swart, J. G., Bertone, G., van Dongen, J., How dark matter came to matter, Nature Astronomy, 2017, 1, 59, 0059, 10.1038/s41550-017-0059,weblink 1703.00013, 2017NatAs...1E..59D, In a talk given in 1884,weblink Lord Kelvin estimated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars orbiting around the center of the galaxy. By using these measurements, he estimated the mass of the galaxy, which he determined is different from the mass of visible stars. Lord Kelvin thus concluded that "many of our stars, perhaps a great majority of them, may be dark bodies".NEWS,weblink A history of dark matter, Ars Technica, 2017-02-08, en-us, In 1906 Henri Poincaré in "The Milky Way and Theory of Gases" used "dark matter", or "matière obscure" in French, in discussing Kelvin's work.The first to suggest the existence of dark matter, using stellar velocities, was Dutch astronomer Jacobus Kapteyn in 1922.JOURNAL, First attempt at a theory of the arrangement and motion of the sidereal system, Jacobus Cornelius, Kapteyn, Jacobus Kapteyn, Astrophysical Journal, 55, 302–327, 1922, It is incidentally suggested that when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect., 1922ApJ....55..302K, 10.1086/142670, (emphasis in original)CONFERENCE, Status of the Axion Dark-Matter Experiment (ADMX), Leslie J, Rosenberg,weblink 30 June 2014, 2, 10th PATRAS Workshop on Axions, WIMPs and WISPs,weblink Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932.Oort, J.H. (1932) "The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems", Bulletin of the Astronomical Institutes of the Netherlands, 6: 249–287.WEB, The Hidden Lives of Galaxies: Hidden Mass, Imagine the Universe!,weblink NASA/GSFC, Oort was studying stellar motions in the local galactic neighborhood and found that the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.JOURNAL, Kuijken, K., Gilmore, G., July 1989, Monthly Notices of the Royal Astronomical Society, 239, 2, 651–664, 1989MNRAS.239..651K, The Mass Distribution in the Galactic Disc – Part III – the Local Volume Mass Density, 10.1093/mnras/239.2.651,weblink In 1933, Swiss astrophysicist Fritz Zwicky, who studied galactic clusters while working at the California Institute of Technology, made a similar inference.JOURNAL, Zwicky, F., Fritz Zwicky, 1933, Die Rotverschiebung von extragalaktischen Nebeln, Helvetica Physica Acta, 6, 110–127, 1933AcHPh...6..110Z, JOURNAL, Zwicky, F., Fritz Zwicky, On the Masses of Nebulae and of Clusters of Nebulae, 1937, The Astrophysical Journal, 86, 217, 1937ApJ....86..217Z, 10.1086/143864, {{Citation |last=Zwicky |first=F. |bibcode=1933AcHPh...6..110Z |title=Die Rotverschiebung von extragalaktischen Nebeln |date=1933 |journal=Helvetica Physica Acta |volume=6 |pages=110–127}} See also {{Citation |last=Zwicky |first=F. |bibcode=1937ApJ....86..217Z |title=On the Masses of Nebulae and of Clusters of Nebulae |date=1937 |journal=Astrophysical Journal |volume=86 |page=217 |doi=10.1086/143864}} Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass that he called dunkle Materie ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated that the cluster had about 400 times more mass than was visually observable. The gravity effect of the visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred that some unseen matter provided the mass and associated gravitation attraction to hold the cluster together. This was the first formal inference about the existence of dark matter.Some details of Zwicky's calculation and of more modern values are given in {{Citation |first=M. |last=Richmond |title=Using the virial theorem: the mass of a cluster of galaxies |url=http://spiff.rit.edu/classes/phys440/lectures/gal_clus/gal_clus.html |accessdate=10 July 2007}} Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant;BOOK, Katherine, Freese, The Cosmic Cocktail: Three Parts Dark Matter, {{google books, y, c2B8AgAAQBAJ, |date=4 May 2014|publisher=Princeton University Press|isbn=978-1-4008-5007-5}} the same calculation today shows a smaller fraction, using greater values for luminous mass. However, Zwicky did correctly infer that the bulk of the matter was dark.{{clarify|date=August 2016}}The first robust indications that the mass to light ratio was anything other than unity came from measurements of galaxy rotation curves. In 1939, Horace W. Babcock reported the rotation curve for the Andromeda nebula (known now as the Andromeda Galaxy), which suggested that the mass-to-luminosity ratio increases radially.Babcock, H, 1939, "The rotation of the Andromeda Nebula", Lick Observatory bulletin; no. 498 He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral and not to missing matter.

1970s

Vera Rubin and Kent Ford in the 1960s and 1970s provided further strong evidence, also using galaxy rotation curves.NEWS, Overbye, Dennis, Dennis Overbye, Vera Rubin, 88, Dies; Opened Doors in Astronomy, and for Women,weblink December 27, 2016, The New York Times, December 27, 2016, First observational evidence of dark matter. Darkmatterphysics.com. Retrieved 6 August 2013. Rubin worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy.JOURNAL, Rubin, Vera C., Vera Rubin, Ford, W. Kent, Jr., Kent Ford (astronomer), February 1970, Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions, The Astrophysical Journal, 159, 379–403, 1970ApJ...159..379R, 10.1086/150317, This result was confirmed in 1978.JOURNAL, Bosma, A., 1978, The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types,weblink Ph.D. Thesis, Rijksuniversiteit Groningen, An influential paper presented Rubin's results in 1980.JOURNAL, Rubin, V., Thonnard, W. K. Jr., Ford, N., 1980, Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R = 4kpc) to UGC 2885 (R = 122kpc), The Astrophysical Journal, 238, 471, 1980ApJ...238..471R, 10.1086/158003, Rubin found that most galaxies must contain about six times as much dark as visible mass;{{sfn|Randall|2015|pp=13–14}} thus, by around 1980 the apparent need for dark matter was widely recognized as a major unsolved problem in astronomy.At the same time that Rubin and Ford were exploring optical rotation curves, radio astronomers were making use of new radio telescopes to map the 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen (HI) often extends to much larger galactic radii than those accessible by optical studies, allowing the sampling of rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of Andromeda with the 300-foot telescope at Green Bank JOURNAL, Roberts, Morton S., Morton S. Roberts (astronomer), May 1966, A High-Resolution 21-cm Hydrogen-Line Survey of the Andromeda Nebula, The Astrophysical Journal, 159, 639–656, 1966ApJ...144..639R, 10.1086/148645, and the 250-foot dish at Jodrell Bank JOURNAL, Gottesman, S. T., S. T. Gottesman (astronomer), Davies, R. D., Rod Davies, Reddish, V. C., Vincent Cartledge Reddish, 1966, A neutral hydrogen survey of the southern regions of the Andromeda nebula, Monthly Notices of the Royal Astronomical Society, 133, 4, 359–387, 1966MNRAS.133..359G, 10.1093/mnras/133.4.359, already showed that the HI rotation curve did not trace the expected Keplerian decline. As more sensitive receivers became available, Morton Roberts and Robert Whitehurst JOURNAL, Roberts, Morton S., Morton S. Roberts (astronomer), Whitehurst, Robert N., Robert N. Whitehurst (astronomer), October 1975, The rotation curve and geometry of M31 at large galactocentric distances, The Astrophysical Journal, 201, 327–346, 1975ApJ...201..327R, 10.1086/153889, were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii, Figure 16 of that paper combines the optical data (the cluster of points at radii of less than 15 kpc with a single point further out) with the HI data between 20 and 30 kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic HI spectroscopy was being developed. In 1972, David Rogstad and Seth ShostakJOURNAL, Rogstad, D. H., David H. Rogstad (astronomer), Shostak, G. Seth, Seth Shostak, September 1972, Gross Properties of Five Scd Galaxies as Determined from 21-centimeter Observations, The Astrophysical Journal, 176, 315–321, 1972ApJ...176..315R, 10.1086/151636, published HI rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended HI disks.A stream of observations in the 1980s supported the presence of dark matter, including gravitational lensing of background objects by galaxy clusters,{{sfn|Randall|2015|pp=14–16}} the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle.JOURNAL, Copi, C. J., Schramm, D. N., Turner, M. S., 1995, Big-Bang Nucleosynthesis and the Baryon Density of the Universe, Science (journal), Science, 267, 5195, 192–199, astro-ph/9407006, 1995Sci...267..192C, 10.1126/science.7809624, 7809624, JOURNAL, Bergstrom, L., 2000, Non-baryonic dark matter: Observational evidence and detection methods, Reports on Progress in Physics, 63, 5, 793–841, hep-ph/0002126, 2000RPPh...63..793B, 10.1088/0034-4885/63/5/2r3, The search for this particle, by a variety of means, is one of the major efforts in particle physics.JOURNAL, Bertone, G., Hooper, D., Silk, J., 2005, Particle dark matter: Evidence, candidates and constraints, Physics Reports, 405, 5–6, 279–390, hep-ph/0404175, 2005PhR...405..279B, 10.1016/j.physrep.2004.08.031,

Technical definition

{{see also|Friedmann equations}}In standard cosmology, matter is anything whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a−3. This is in contrast to radiation, which scales to the inverse fourth power of the scale factor ρ ∝ a−4, and dark energy, which is unaffected ρ ∝ a0. This can be understood intuitively: for an ordinary particle in a square box, doubling the length of the sides of the box decreases the density (and hence energy density) by a factor of eight (23). For radiation, the decrease in energy density is greater, because an increase in spatial distance also causes a redshift. Dark energy, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.WEB,weblink Cosmology: Part III Mathematical Tripos, Cambridge University, Daniel Baumann, 21−22, Dark matter is that component of the universe that is not ordinary matter but still obeys ρ ∝ a−3.

Observational evidence

File:Artist’s impression of the expected dark matter distribution around the Milky Way.ogv|thumb|This artist’s impression shows the expected distribution of dark matter in the Milky Way galaxy as a blue halo of material surrounding the galaxy.PRESS, 18 April 2012, Serious Blow to Dark Matter Theories?,weblink European Southern ObservatoryEuropean Southern Observatory

Galaxy rotation curves

File:GalacticRotation2.svg|thumb|(Galaxy rotation curve|Rotation curve]] of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the 'flat' appearance of the velocity curve out to a large radius.)(File:Comparison of rotating disc galaxies in the distant Universe and the present day.webm|thumb|Models of rotating disc galaxies in the present day (left) and ten billion years ago (right). In the present-day galaxy, dark matter - shown in red - is more concentrated near the center and it rotates more rapidly (effect exaggerated).)The arms of spiral galaxies rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the solar system.This is a consequence of the shell theorem and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D). From Kepler's Second Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.JOURNAL, Corbelli, E. & Salucci, P., The extended rotation curve and the dark matter halo of M33, Monthly Notices of the Royal Astronomical Society, 2000, 311, 2, 441–447, 10.1046/j.1365-8711.2000.03075.x, astro-ph/9909252, 2000MNRAS.311..441C, Instead, the galaxy rotation curve remains flat as distance from the center increases.If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude that the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

Velocity dispersions

Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxiesJOURNAL, Faber, S. M., Jackson, R. E., 1976, Velocity dispersions and mass-to-light ratios for elliptical galaxies, The Astrophysical Journal, 204, 668–683, 1976ApJ...204..668F, 10.1086/154215, do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.BOOK, Binny, James, Merrifield, Michael, 1998, Galactic Astronomy, Princeton University Press, 712-713, As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.

Galaxy clusters

File:Gravitationell-lins-4.jpg|thumb|upright|Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689Abell 1689Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:
  • From the scatter in radial velocities of the galaxies within clusters
  • From X-rays emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
  • Gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).
Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.JOURNAL, Cosmological Parameters from Clusters of Galaxies, 10.1146/annurev-astro-081710-102514, 1103.4829, 2011ARA&A..49..409A,

Gravitational lensing

(File:Dark matter map of KiDS survey region (region G12).jpg|left|thumb|Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree survey.WEB, Dark Matter May be Smoother than Expected – Careful study of large area of sky imaged by VST reveals intriguing result,weblink www.eso.org, 8 December 2016, )One of the consequences of general relativity is that massive objects (such as a cluster of galaxies) lying between a more distant source (such as a quasar) and an observer should act as a lens to bend the light from this source. The more massive an object, the more lensing is observed.Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including Abell 1689.JOURNAL, Taylor, A. N., etal, 1998, Gravitational Lens Magnification and the Mass of Abell 1689, The Astrophysical Journal, 501, 2, 539–553, astro-ph/9801158, 1998ApJ...501..539T, 10.1086/305827, By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.JOURNAL, Wu, X., Chiueh, T., Fang, L., Xue, Y., 1998, A comparison of different cluster mass estimates: consistency or discrepancy?, Monthly Notices of the Royal Astronomical Society, 301, 3, 861–871, astro-ph/9808179, 1998MNRAS.301..861W, 10.1046/j.1365-8711.1998.02055.x, Lensing can lead to multiple copies of an image. By analyzing the distribution of multiple image copies, scientists have been able to deduce and map the distribution of dark matter around the MACS J0416.1-2403 galaxy cluster.JOURNAL, Cho, Adrian, Scientists unveil the most detailed map of dark matter to date, Science, 2017, 10.1126/science.aal0847,weblink JOURNAL, Natarajan, Priyamvada, Chadayammuri, Urmila, Jauzac, Mathilde, Richard, Johan, Kneib, Jean-Paul, Ebeling, Harald, Jiang, Fangzhou, Bosch, Frank van den, Limousin, Marceau, Jullo, Eric, Atek, Hakim, Pillepich, Annalisa, Popa, Cristina, Marinacci, Federico, Hernquist, Lars, Meneghetti, Massimo, Vogelsberger, Mark, Mapping substructure in the HST Frontier Fields cluster lenses and in cosmological simulations, Monthly Notices of the Royal Astronomical Society, 468, 2, 1962, 2017, 10.1093/mnras/stw3385,weblink 1702.04348, 2017MNRAS.468.1962N, Weak gravitational lensing investigates minute distortions of galaxies, using statistical analyses from vast galaxy surveys. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.JOURNAL, Refregier, A., 2003, Weak gravitational lensing by large-scale structure, Annual Review of Astronomy and Astrophysics, 41, 1, 645–668, astro-ph/0307212, 2003ARA&A..41..645R, 10.1146/annurev.astro.41.111302.102207, Dark matter does not bend light itself; mass (in this case the mass of the dark matter) bends spacetime. Light follows the curvature of spacetime, resulting in the lensing effect.Quasars, lensing, and dark matter. Physics for the 21st Century. Annenberg Foundation. 2017.Hubble snaps dark matter warping spacetime. Rik Myslewski, The Register. 14 Oct 2011.

Cosmic microwave background

(File:WMAP 2008 universe content.png|thumb|upright|Estimated division of total energy in the universe into matter, dark matter and dark energy based on five years of WMAP data.WEB, Content of the Universe - Pie Chart,weblink Wilkinson Microwave Anisotropy Probe, National Aeronautics and Space Administration, 9 January 2018, )Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via Thomson scattering. Dark matter does not interact directly with radiation, but it does affect the CMB by its gravitational potential (mainly on large scales), and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the cosmic microwave background (CMB).The cosmic microwave background is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The series of peaks can be predicted for any assumed set of cosmological parameters by modern computer codes such as CMBFast and CAMB, and matching theory to data, therefore, constrains cosmological parameters.The details are technical. See WEB, Wayne Hu, Intermediate Guide to the Acoustic Peaks and Polarization,weblink 2001, for an intermediate-level introduction. The first peak mostly shows the density of baryonic matter, while the third peak relates mostly to the density of dark matter, measuring the density of matter and the density of atoms.The CMB anisotropy was first discovered by COBE in 1992, though this had too coarse resolution to detect the acoustic peaks. After the discovery of the first acoustic peak by the balloon-borne BOOMERanG experiment in 2000, the power spectrum was precisely observed by WMAP in 2003-12, and even more precisely by the Planck spacecraft in 2013-15. The results support the Lambda-CDM model.JOURNAL, Hinshaw, G., etal, 2009, Five-Year Wilkinson Microwave Anisotropy Probe Observations: Data Processing, Sky Maps, and Basic Results, The Astrophysical Journal Supplement, 180, 2, 225–245, 0803.0732, 2009ApJS..180..225H, 10.1088/0067-0049/180/2/225, The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the Lambda-CDM model but difficult to reproduce with any competing model such as MOND.JOURNAL, Skordis et al., C., Large Scale Structure in Bekenstein's theory of relativistic Modified Newtonian Dynamics, Phys. Rev. Lett., 2006, 96, 011301, 10.1103/PhysRevLett.96.011301, 2006PhRvL..96a1301S, astro-ph/0505519, JOURNAL, Ade et al., P.A.R., Planck 2015 results. XIII. Cosmological parameters, Astron. Astrophys., 2015, 594, A13, 10.1051/0004-6361/201525830, 2016A&A...594A..13P, 1502.01589,

Structure formation

File:COSMOS 3D dark matter map.png|thumb|Mass map|3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.PRESS, 7 January 2007, Hubble Maps the Cosmic Web of "Clumpy" Dark Matter in 3-D,weblink NASANASAStructure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.WEB, A. H. Jaffe, Cosmology 2012: Lecture Notes,weblink If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process.JOURNAL,weblink "Constraints on the composite photon theory, L. F. Low, 10.1142/S021773231675002X, Modern Physics Letters A, 12 October 2016, 2016MPLA...3175002L,

Bullet Cluster

If dark matter does not exist, then the next most likely explanation is that general relativity – the prevailing theory of gravity – is incorrect. The Bullet Cluster, the result of a recent collision of two galaxy clusters, provides a challenge for modified gravity theories because its apparent center of mass is far displaced from the baryonic center of mass.JOURNAL
, Clowe
, Douglas
, 2006
, A Direct Empirical Proof of the Existence of Dark Matter
, The Astrophysical Journal Letters
, 648
, 2
, L109–L113
, astro-ph/0608407
, 10.1086/508162
, 2006ApJ...648L.109C
PUBLISHER=ARS TECHNICADATE=21 SEPTEMBER 2017, HTTPS://WWW.FORBES.COM/SITES/STARTSWITHABANG/2017/11/09/THE-BULLET-CLUSTER-PROVES-DARK-MATTER-EXISTS-BUT-NOT-FOR-THE-REASON-MOST-PHYSICISTS-THINK/#3032B6081738>TITLE=THE BULLET CLUSTER PROVES DARK MATTER EXISTS, BUT NOT FOR THE REASON MOST PHYSICISTS THINKAUTHOR=ETHAN SIEGELAUTHOR=M. MARKEVITCHAUTHOR2=S. RANDALL AUTHOR4=A. GONZALEZ LAST-AUTHOR-AMP=YES DATE=16–23 JULY 2006, Beijing, China, Abstract only

Type Ia supernova distance measurements

Type Ia supernovae can be used as standard candles to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past. The data indicates that the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy.JOURNAL, Kowalski, M., etal, 2008, Improved Cosmological Constraints from New, Old, and Combined Supernova Data Sets, The Astrophysical Journal, 686, 2, 749–778, 0804.4142, 2008ApJ...686..749K, 10.1086/589937, Since observations indicate the universe is almost flat,WEB, Will the Universe expand forever?,weblink NASA, 24 January 2014, 16 March 2015, WEB, Our universe is Flat,weblink FermiLab/SLAC, 7 April 2015, JOURNAL, Unexpected connections, Marcus Y. Yoo, Engineering & Science, Caltech, LXXIV1, 2011, 30, it is expected that the total energy density of everything in the universe to sum to 1 (Ωtot ~ 1). The measured dark energy density is ΩΛ = ~0.690; the observed ordinary (baryonic) matter energy density is Ωb = ~0.0482 and the energy density of radiation is negligible. This leaves a missing Ωdm = ~0.258 that nonetheless behaves like matter (see technical definition section above) – dark matter.WEB,weblink Planck Publications: Planck 2015 Results, European Space Agency, February 2015, 9 February 2015,

Sky surveys and baryon acoustic oscillations

Baryon acoustic oscillations (BAO) are regular, periodic fluctuations in the density of the visible baryonic matter (normal matter) of the universe. These are predicted to arise in the Lambda-CDM model due to the early universe's acoustic oscillations in the photon-baryon fluid and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (~ 1 percent) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130 or 160 Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.JOURNAL, Percival, W. J., etal, 2007, Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, Monthly Notices of the Royal Astronomical Society, 381, 3, 1053–1066, 0705.3323, 2007MNRAS.381.1053P, 10.1111/j.1365-2966.2007.12268.x, Combining the CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe.JOURNAL, Komatsu, E., etal, 2009, Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation, The Astrophysical Journal Supplement, 180, 2, 330–376, 0803.0547, 2009ApJS..180..330K, 10.1088/0067-0049/180/2/330, The results support the Lambda-CDM model.

Redshift-space distortions

Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding but more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. The effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures assuming Earth is not at a special location in the Universe.The effect was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the 2dF Galaxy Redshift Survey.JOURNAL, Peacock, J., A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey, Nature, 2001, 410, 6825, 169–73, astro-ph/0103143, 2001Natur.410..169P, etal, 10.1038/35065528, 11242069, Results are in agreement with the Lambda-CDM model.

Lyman-alpha forest

In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models.JOURNAL, Viel, M., Bolton, J. S., Haehnelt, M. G., 2009, Cosmological and astrophysical constraints from the Lyman α forest flux probability distribution function, Monthly Notices of the Royal Astronomical Society, 399, 1, L39–L43, 0907.2927, 2009MNRAS.399L..39V, 10.1111/j.1745-3933.2009.00720.x, These constraints agree with those obtained from WMAP data.

Composition of dark matter: baryonic vs. nonbaryonic

{{unsolved|physics|What is dark matter? How is it generated? Is it related to supersymmetry?}}Dark matter can refer to any substance that interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons.{{refn|name=BaryonicFootnote01|group=note|Astronomers define the term baryonic matter to refer to ordinary matter made of protons, neutrons and electrons, including neutron stars and black holes from the collapse of ordinary matter. Strictly speaking, electrons are leptons not baryons; but since their number is equal to the protons while their mass is far smaller, electrons give a negligible contribution to the average density of baryonic matter. Baryonic matter excludes other known particles such as photons and neutrinos. Hypothetical primordial black holes are also generally defined as non-baryonic, since they would have formed from radiation, not matter.WEB,weblink Baryonic Matter, COSMOS - The SAO Encyclopedia of Astronomy, Swinburne University of Technology, 9 April 2018, }} However, for the reasons outlined below, most scientists think the dark matter is dominated by a non-baryonic component, which is likely composed of a currently unknown fundamental particle (or similar exotic state).File:Fermi Observations of Dwarf Galaxies Provide New Insights on Dark Matter.ogv|thumb|Fermi-LAT observations of dwarf galaxies provide new insights on dark matter.]]

Baryonic matter

{{Distinguish|Missing baryon problem}}Baryons (protons and neutrons) make up ordinary stars and planets. However, baryonic matter also encompasses less common black holes, neutron stars, faint old white dwarfs and brown dwarfs, collectively known as massive compact halo objects (MACHOs), which can be hard to detect.{{sfn|Randall|2015|p=286}}However multiple lines of evidence suggest the majority of dark matter is not made of baryons:
  • Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
  • The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.Achim Weiss, "Big Bang Nucleosynthesis: Cooking up the first light elements" in: Einstein Online Vol. 2 (2006), 1017BOOK, Raine, D., Thomas, T., 2001, An Introduction to the Science of Cosmology,weblink 30, IOP Publishing, 0-7503-0405-7, bot: unknown,weblink" title="web.archive.org/web/20130530073650weblink">weblink 30 May 2013, dmy-all, Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.
  • Astronomical searches for gravitational microlensing in the Milky Way found that at most a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30 solar masses, which covers nearly all the plausible candidates.JOURNAL, Tisserand, P., Le Guillou, L., Afonso, C., Albert, J. N., Andersen, J., Ansari, R., Aubourg, É., Bareyre, P., Beaulieu, J. P., Charlot, 10.1051/0004-6361:20066017, X., Coutures, C., Ferlet, R., Fouqué, P., Glicenstein, J. F., Goldman, B., Gould, A., Graff, D., Gros, M., Haissinski, J., Hamadache, C., De Kat, J., Lasserre, T., Lesquoy, É., Loup, C., Magneville, C., Marquette, J. B., Maurice, É., Maury, A., Milsztajn, A., Moniez, M., Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds, Astronomy and Astrophysics, 469, 2, 387–404, 2007,weblink astro-ph/0607207, 2007A&A...469..387T, JOURNAL, Graff, D. S., Freese, K., Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo, The Astrophysical Journal, 456, L49, 1996, astro-ph/9507097, 1996ApJ...456L..49G, 10.1086/309850, JOURNAL, Najita, J. R., Tiede, G. P., Carr, J. S., 10.1086/309477, From Stars to Superplanets: The Low‐Mass Initial Mass Function in the Young Cluster IC 348, The Astrophysical Journal, 541, 2, 977–1003, 2000, astro-ph/0005290, 2000ApJ...541..977N, Wyrzykowski, Lukasz et al. (2011) The OGLE view of microlensing towards the Magellanic Clouds – IV. OGLE-III SMC data and final conclusions on MACHOs, MNRAS, 416, 2949ARXIV


, Death of Stellar Baryonic Dark Matter Candidates
, Katherine
, Freese
, astro-ph/0007444


, Fields
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, Graff
, David
, 2000
, astro-ph, BOOK, The First Stars, Katherine, Freese, astro-ph/0002058, 2000fist.conf...18F, 10.1007/10719504_3, Death of Stellar Baryonic Dark Matter, ESO Astrophysics Symposia, 2000, Fields, Brian, Graff, David, 3-540-67222-2, 18,
  • Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background.JOURNAL, L., Canetti, M., Drewes, M., Shaposhnikov, Matter and Antimatter in the Universe, New J. Phys., 2012, 14, 9, 095012, 10.1088/1367-2630/14/9/095012, 1204.4186, 2012NJPh...14i5012C, Observations by WMAP and Planck indicate that around five-sixths of the total matter is in a form that interacts significantly with ordinary matter or photons only through gravitational effects.

Non-baryonic matter

Candidates for non-baryonic dark matter are hypothetical particles such as axions, sterile neutrinos, weakly-interacting massive particles (WIMPs), gravitationally-interacting massive particles (GIMPs), or supersymmetric particles. The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses – however uncertain they may be – are almost certainly tiny, they can only supply a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.Unlike baryonic matter, nonbaryonic matter did not contribute to the formation of the elements in the early universe (Big Bang nucleosynthesis) and so its presence is revealed only via its gravitational effects, or, weak lensing. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves, possibly resulting in observable by-products such as gamma rays and neutrinos (indirect detection).JOURNAL, Bertone, G., Merritt, D., Dark Matter Dynamics and Indirect Detection, 2005, Modern Physics Letters A, 20, 14, 1021–1036, astro-ph/0504422, 2005MPLA...20.1021B, 10.1142/S0217732305017391,

Dark matter aggregation and dense dark matter objects

If dark matter is as common as observations suggest, an obvious question is whether it can form objects equivalent to planets, stars, or black holes. The answer has historically been that it cannot,Synopsis: A Way to Cool Dark Matter - American Physical Society: "One widely held belief about dark matter is that it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way that baryonic matter forms planets, stars, and galaxies. Observations so far suggest that dark matter doesn't do that—it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) dark matter."Why Doesn't Dark Matter Form Black Holes? - Forbes column, authored by Ethan Siegel, whose Forbes biography identifies the author as a "Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy... numerous awards for science writing ... including the award for best science blog by the Institute of Physics. because of two factors:
  • It lacks an efficient means to lose energy: Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase velocity and momentum. Dark matter appears to lack means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The Virial theorem suggests that such a particle would not stay bound to the gradually forming object - as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.
  • It lacks a range of interactions needed to form structures: Ordinary matter interacts in many different ways. This allow it to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of neutrinos and electromagnetic radiation through fusion when they become energetic enough. Protons and neutrons can bind via the strong interaction and then form atoms with electrons largely through electromagnetic interaction. But there is no evidence that dark matter is capable of such a wide variety of interactions, since it only seems to interact through gravity and through some means no stronger than the weak interaction. (although this is speculative until dark matter is better understood).
This question has been debated heavily during recent years. In 2016–2017 the idea of dense dark matter or dark matter being black holes, including primordial black holes, made a comebacweblinkweblink following results of gravitation wave detection. These were again ruled out in December 2017weblink but research and theories based on these still continue as at 2018, including approaches to dark matter coolingweblink and the question is by no means settled.

Classification of dark matter: cold, warm or hot

Dark matter can be divided into cold, warm, and hot categories.BOOK, Joseph, Silk, The Big Bang: Third Edition, {{google books, y, XLwe1lUmz5kC, 82, | date=6 December 2000| publisher=Henry Holt and Company| isbn=978-0-8050-7256-3| chapter=IX}} These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion â€“ this is an important distance called the free streaming length (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation. The categories are set with respect to the size of a protogalaxy (an object that later evolves into a dwarf galaxy): dark matter particles are classified as cold, warm, or hot according as their FSL; much smaller (cold), similar (warm), or much larger (hot) than a protogalaxy.JOURNAL, N., Vittorio, J. Silk, 1984, Astrophysical Journal Letters, 285, L39–L43, Fine-scale anisotropy of the cosmic microwave background in a universe dominated by cold dark matter, 10.1086/184361, 1984ApJ...285L..39V, JOURNAL, Masayuki, Umemura, Satoru Ikeuchi, Formation of Subgalactic Objects within Two-Component Dark Matter, 1985, Astrophysical Journal, 299, 583–592, 10.1086/163726, 1985ApJ...299..583U, Mixtures of the above are also possible: a theory of mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of dark energy.{{Citation needed|date=April 2016}}Cold dark matter leads to a bottom-up formation of structure while hot dark matter would result in a top-down formation scenario; the latter is excluded by high-redshift galaxy observations.

Alternative definitions

These categories also correspond to fluctuation spectrum effects and the interval following the Big Bang at which each type became non-relativistic. Davis et al. wrote in 1985:JOURNAL, Davis, M., Efstathiou, G., Frenk, C. S., & White, S. D. M., The evolution of large-scale structure in a universe dominated by cold dark matter, Astrophysical Journal, 292, May 15, 1985, 371–394, 10.1086/163168, 1985ApJ...292..371D, Another approximate dividing line is that warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era (photons and neutrinos), with a photon temperature 2.7 million K. Standard physical cosmology gives the particle horizon size as 2ct (speed of light multiplied by time) in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light years today (absent structure formation). The actual FSL is approximately 5 times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic. In this example the FSL would correspond to 10 million light-years or 3 Mpc today, around the size containing an average large galaxy.The 2.7 million K photon temperature gives a typical photon energy of 250 electron-volts, thereby setting a typical mass scale for warm dark matter: particles much more massive than this, such as GeV â€“ TeV mass WIMPs, would become non-relativistic much earlier than 1 year after the Big Bang and thus have FSLs much smaller than a protogalaxy, making them cold. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, thus qualifying them as hot.

Cold dark matter

Cold dark matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early.The constituents of cold dark matter are unknown. Possibilities range from large objects like MACHOs (such as black holesJOURNAL, Hawkins, M. R. S., The case for primordial black holes as dark matter, 10.1111/j.1365-2966.2011.18890.x, Monthly Notices of the Royal Astronomical Society, 415, 3, 2744–2757, 2011, 1106.3875, 2011MNRAS.415.2744H, ) or RAMBOs (such as clusters of brown dwarfs), to new particles such as WIMPs and axions.Studies of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologistsJOURNAL, 10.1155/2011/968283, MACHOs can only account for a very small percentage of the nonluminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no..., Dark Matter: A Primer, 2011, Garrett, Katherine, Dūda, Gintaras, Advances in Astronomy, 2011, 1–22, 1006.2483, 2011AdAst2011E...8G, JOURNAL, Bertone, G., The moment of truth for WIMP dark matter, 10.1038/nature09509, Nature, 468, 7322, 389–393, 2010, 21085174, 1011.3532, 2010Natur.468..389B, Olive, Keith A. (2003). "TASI Lectures on Dark Matter". p. 21 that MACHOsJOURNAL, Carr, B. J., etal, May 2010, New cosmological constraints on primordial black holes, Physical Review D, 81, 104019, 10.1103/PhysRevD.81.104019,weblink 0912.5297, 2010PhRvD..81j4019C, 10, cannot make up more than a small fraction of dark matter. According to A. Peter: "... the only really plausible dark-matter candidates are new particles."ARXIV
, Peter
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, Dark Matter: A Brief Review
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The 1997 DAMA/NaI experiment and its successor DAMA/LIBRA in 2013, claimed to directly detect dark matter particles passing through the Earth, but many researchers remain skeptical, as negative results from similar experiments seem incompatible with the DAMA results.Many supersymmetric models offer dark matter candidates in the form of the WIMPy Lightest Supersymmetric Particle (LSP).JOURNAL, Supersymmetric dark matter,weblink Physics Reports, 1 March 1996, 195–373, 267, 5–6, 10.1016/0370-1573(95)00058-5, Gerard, Jungman, Marc, Kamionkowski, Kim, Griest, hep-ph/9506380, 1996PhR...267..195J, Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.

Warm dark matter

Warm dark matter comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Some researchers consider this a better fit to observations. A challenge for this model is the lack of particle candidates with the required mass ~ 300 eV to 3000 eV.{{Citation needed|date=April 2016}}No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: a heavier, slower form of neutrino that does not interact through the weak force, unlike other neutrinos. Some modified gravity theories, such as scalar-tensor-vector gravity, require "warm" dark matter to make their equations work.

Hot dark matter

Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as such particle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and detected in 1956. Neutrinos' mass is less than 10−6 that of an electron. Neutrinos interact with normal matter only via gravity and the weak force, making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on). This makes them 'weakly-interacting light particles' (WILPs), as opposed to WIMPs.The three known flavours of neutrinos are the electron, muon, and tau. Their masses are slightly different. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos (or for any of the three individually). For example, if the average neutrino mass were over 50 eV/c2 (less than 10−5 of the mass of an electron), the universe would collapse. CMB data and other methods indicate that their average mass probably does not exceed 0.3 eV/c2. Thus, observed neutrinos cannot explain dark matter.WEB,weblink Neutrinos as Dark Matter, Astro.ucla.edu, 21 September 1998, 6 January 2011, Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies that the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies. Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together.

Detection of dark matter particles

If dark matter is made up of sub-atomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.JOURNAL, Gaitskell, Richard J., Direct Detection of Dark Matter, Annual Review of Nuclear and Particle Science, 54, 315–359, 2004ARNPS..54..315G, 2004, 10.1146/annurev.nucl.54.070103.181244, WEB, NEUTRALINO DARK MATTER,weblink 26 December 2011, WEB, WIMPs and MACHOs,weblink Griest, Kim, 26 December 2011, Many experiments aim to test this hypothesis. Although WIMPs are popular search candidates, the Axion Dark Matter eXperiment (ADMX) searches for axions. Another candidate is heavy hidden sector particles that only interact with ordinary matter via gravity.These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.

Direct detection

{{details|Weakly-interacting massive particles#Direct detection}}Direct detection experiments aim to observe low-energy recoils (typically a few keVs) of nuclei induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil the nucleus will emit energy as, e.g., scintillation light or phonons, which is then detected by sensitive apparatus. To do this effectively it is crucial to maintain a low background, and so such experiments operate deep underground to reduce the interference from cosmic rays. Examples of underground laboratories with direct detection experiments include the Stawell mine, the Soudan mine, the SNOLAB underground laboratory at Sudbury, the Gran Sasso National Laboratory, the Canfranc Underground Laboratory, the Boulby Underground Laboratory, the Deep Underground Science and Engineering Laboratory and the China Jinping Underground Laboratory.These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include: CDMS, CRESST, EDELWEISS, EURECA. Noble liquid experiments include ZEPLIN, XENON, DEAP, ArDM, WARP, DarkSide, PandaX, and LUX, the Large Underground Xenon experiment. Both of these techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include SIMPLE and PICASSO.Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.JOURNAL, Drees, M., Gerbier, G., Dark Matter, Chin. Phys. C, 2015, 38, 090001,weblink The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations claim to have detected an annual modulation in the rate of events in their detectors,JOURNAL, Bernabei, R., First results from DAMA/LIBRA and the combined results with DAMA/NaI, Eur. Phys. J. C, 56, 3, 333–355, 2008, 10.1140/epjc/s10052-008-0662-y, 0804.2741, Belli, P., Cappella, F., Cerulli, R., Dai, C. J., d’Angelo, A., He, H. L., Incicchitti, A., Kuang, H. H., Ma, J. M., Montecchia, F., Nozzoli, F., Prosperi, D., Sheng, X. D., Ye, Z. P., 2008EPJC...56..333B, JOURNAL, 10.1103/PhysRevD.33.3495, Drukier, A., Freese, K., Spergel, D., Detecting Cold Dark Matter Candidates, Physical Review D, 33, 12, 3495–3508, 1986, 1986PhRvD..33.3495D, which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX and SuperCDMS.JOURNAL, Davis, Jonathan H., The Past and Future of Light Dark Matter Direct Detection, Int. J. Mod. Phys. A, 2015, 30, 15, 1530038, 10.1142/S0217751X15300380, 1506.03924, 2015IJMPA..3030038D, A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center.NEWS, Stonebraker, Alan, Synopsis: Dark-Matter Wind Sways through the Seasons, Physics – Synopses, American Physical Society, 3 January 2014,weblink 6 January 2014, JOURNAL, Lee, Samuel K., Mariangela Lisanti, Annika H. G. Peter, and Benjamin R. Safdi, 10.1103/PhysRevLett.112.011301, 1308.1953, Effect of Gravitational Focusing on Annual Modulation in Dark-Matter Direct-Detection Experiments, 3 January 2014, Phys. Rev. Lett., 112, 1, 011301 (2014) [5 pages], American Physical Society, 2014PhRvL.112a1301L, 24483881, NEWS, The Dark Matter Group, An Introduction to Dark Matter, Dark Matter Research, Sheffield, UK, University of Sheffield,weblink 7 January 2014, NEWS, Scientists at Kavli MIT are working on...a tool to track the movement of dark matter., Blowing in the Wind, Kavli News, Sheffield, UK, Kavli Foundation (United States), Kavli Foundation,weblink 7 January 2014, A low pressure time projection chamber makes it possible to access information on recoiling tracks and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun traveles (approximately towards Cygnus) may then be separated from background, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

Indirect detection

(File:Collage of six cluster collisions with dark matter maps.jpg|thumb|Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.WEB, Dark matter even darker than once thought,weblink 16 June 2015, )File:Turning Black Holes into Dark Matter Labs.webm|thumb|Video about the potential gamma-ray detection of dark matter annihilation around (supermassive black hole]]s. (Duration 3:13, also see file description.))Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the centre of our galaxy) two dark matter particles could annihilate to produce gamma rays or Standard Model particle-antiparticle pairs.BOOK, Gianfranco, Bertone, Particle Dark Matter: Observations, Models and Searches,weblink 2010, Cambridge University Press, 83–104, Dark Matter at the Centers of Galaxies, 1001.3706, 978-0-521-76368-4, 2010arXiv1001.3706M, Alternatively if the dark matter particle is unstable, it could decay into standard model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions in our galaxy or others.JOURNAL, Ellis, J., Flores, R. A., Freese, K., Ritz, S., Seckel, D., Silk, J., 10.1016/0370-2693(88)91385-8, Cosmic ray constraints on the annihilations of relic particles in the galactic halo, Physics Letters B, 214, 3, 403–412, 1988, 1988PhLB..214..403E, A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy neutrinos.JOURNAL, 10.1016/0370-2693(86)90349-7, Freese, K., Can Scalar Neutrinos or Massive Dirac Neutrinos be the Missing Mass?, Physics Letters B, 167, 3, 295–300, 1986, 1986PhLB..167..295F, Such a signal would be strong indirect proof of WIMP dark matter. High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.{{sfn|Randall|2015|p=298}}The detection by LIGO in September 2015 of gravitational waves, opens the possibility of observing dark matter in a new way, particularly if it is in the form of primordial black holes.JOURNAL,weblink Surfing Gravity's Waves, New Scientist, Joshua, Sokol, etal, 3061, 20 February 2016, WEB, Johns Hopkins University, Did Gravitational Wave Detector Find Dark Matter?, 15 June 2016,weblink 20 June 2015, While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery. Because there’s so little evidence of them, though, the primordial black hole-dark matter hypothesis has not gained a large following among scientists. The LIGO findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter. Predictions made by scientists in the past held that conditions at the birth of the universe would produce lots of these primordial black holes distributed approximately evenly in the universe, clustering in halos around galaxies. All this would make them good candidates for dark matter., JOURNAL, Bird, Simeon, Cholis, Illian, 19 May 2016, Did LIGO Detect Dark Matter?,weblink Physical Review Letters, 116, 201301, 20, 201301, 10.1103/PhysRevLett.116.201301, 20 June 2016, 2016PhRvL.116t1301B, 1603.00464, Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow.The Energetic Gamma Ray Experiment Telescope observed more gamma rays in 2008 than expected from the Milky Way, but scientists concluded that this was most likely due to incorrect estimation of the telescope's sensitivity.JOURNAL, F.W., Stecker, The likely cause of the EGRET GeV anomaly and its implications, Astroparticle Physics, 29, 1, 2008, 25–29, 10.1016/j.astropartphys.2007.11.002, 0705.4311, Hunter, S, Kniffen, D, 2008APh....29...25S, The Fermi Gamma-ray Space Telescope is searching for similar gamma rays.JOURNAL, W.B., Atwood, The large area telescope on the Fermi Gamma-ray Space Telescope Mission, Astrophysical Journal, 697, 2, 2009, 1071–1102, 10.1088/0004-637X/697/2/1071, 0902.1089, Abdo, A. A., Ackermann, M., Althouse, W., Anderson, B., Axelsson, M., Baldini, L., Ballet, J., Band, D. L., 2009ApJ...697.1071A, 8, In April 2012, an analysis of previously available data from its Large Area Telescope instrument produced statistical evidence of a 130 GeV signal in the gamma radiation coming from the center of the Milky Way.JOURNAL, 10.1088/1475-7516/2012/08/007, Weniger, Christoph, A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope, Journal of Cosmology and Astroparticle Physics, 8, 2012, 1204.2797v2, 2012, 7, 2012JCAP...08..007W, WIMP annihilation was seen as the most probable explanation.WEB,weblink Gamma rays hint at dark matter, Cartlidge, Edwin, 24 April 2012, Institute Of Physics, 23 April 2013, At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxiesJOURNAL, Albert, J., Aliu, E., Anderhub, H., Antoranz, P., Backes, M., Baixeras, C., Barrio, J. A., Bartko, H., Bastieri, D., Becker, J. K., Bednarek, W., Berger, K., Bigongiari, C., Biland, A., Bock, R. K., Bordas, P., Bosch‐Ramon, V., Bretz, T., Britvitch, I., Camara, M., Carmona, E., Chilingarian, A., Commichau, S., Contreras, J. L., Cortina, J., Costado, M. T., Curtef, V., Danielyan, V., Dazzi, F., De Angelis, A., Upper Limit for γ‐Ray Emission above 140 GeV from the Dwarf Spheroidal Galaxy Draco, 10.1086/529135, The Astrophysical Journal, 679, 428–431, 2008, 0711.2574, 2008ApJ...679..428A, 29, and in clusters of galaxies.JOURNAL, Aleksić, J., Antonelli, L. A., Antoranz, P., Backes, M., Baixeras, C., Balestra, S., Barrio, J. A., Bastieri, D., González, J. B., Bednarek, W., Berdyugin, A., Berger, K., Bernardini, E., Biland, A., Bock, R. K., Bonnoli, G., Bordas, P., Tridon, D. B., Bosch-Ramon, V., Bose, D., Braun, I., Bretz, T., Britzger, D., Camara, M., Carmona, E., Carosi, A., Colin, P., Commichau, S., Contreras, J. L., Cortina, J., Magic Gamma-Ray Telescope Observation of the Perseus Cluster of Galaxies: Implications for Cosmic Rays, Dark Matter, and Ngc 1275, 10.1088/0004-637X/710/1/634, The Astrophysical Journal, 710, 634–647, 2010, 0909.3267, 2010ApJ...710..634A, 29, The PAMELA experiment (launched 2006) detected excess positrons. They could be from dark matter annihilation or from pulsars. No excess antiprotons were observed.JOURNAL, Adriani, O., Barbarino, G. C., Bazilevskaya, G. A., Bellotti, R., Boezio, M., Bogomolov, E. A., Bonechi, L., Bongi, M., Bonvicini, V., Bottai, 10.1038/nature07942, S., Bruno, A., Cafagna, F., Campana, D., Carlson, P., Casolino, M., Castellini, G., De Pascale, M. P., De Rosa, G., De Simone, N., Di Felice, V., Galper, A. M., Grishantseva, L., Hofverberg, P., Koldashov, S. V., Krutkov, S. Y., Kvashnin, A. N., Leonov, A., Malvezzi, V., Marcelli, L., Menn, W., An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV, Nature, 458, 7238, 607–609, 2009, 19340076, 0810.4995, 2009Natur.458..607A, In 2013 results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays that could be due to dark matter annihilation.JOURNAL, First Result from the Alpha Magnetic Spectrometer on the International Space Station: Precision Measurement of the Positron Fraction in Primary Cosmic Rays of 0.5–350 GeV,weblink 3 April 2013, Physical Review Letters, 3 April 2013, Aguilar, M. (AMS Collaboration), 10.1103/PhysRevLett.110.141102, 2013PhRvL.110n1102A, etal, 110, 14, 25166975, 141102, WEB, First Result from the Alpha Magnetic Spectrometer Experiment,weblink 3 April 2013, AMS Collaboration, 3 April 2013, NEWS, Heilprin, John, Borenstein, Seth, Scientists find hint of dark matter from cosmos,weblink 3 April 2013, Associated Press, 3 April 2013, NEWS, Amos, Jonathan, Alpha Magnetic Spectrometer zeroes in on dark matter,weblink 3 April 2013, BBC, 3 April 2013, WEB, Perrotto, Trent J., Byerly, Josh, NASA TV Briefing Discusses Alpha Magnetic Spectrometer Results,weblink 2 April 2013, NASA, 3 April 2013, NEWS, Overbye, Dennis, New Clues to the Mystery of Dark Matter,weblink 3 April 2013, The New York Times, 3 April 2013,

Collider searches for dark matter

An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect dark matter particles produced in collisions of the LHC proton beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected.JOURNAL, Kane, G., Watson, S., Dark Matter and LHC:. what is the Connection?, Modern Physics Letters A, 2008, 23, 2103–2123, 10.1142/S0217732308028314, 2008MPLA...23.2103K, 0807.2244, 26, Constraints on dark matter also exist from the LEP experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.JOURNAL, Fox, P.J., Harnik, R., Kopp, J., Tsai, Y., LEP Shines Light on Dark Matter, Phys. Rev. D, 2011, 84, 014028, 10.1103/PhysRevD.84.014028,weblink 1103.0240, 2011PhRvD..84a4028F, It is important to note that any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, the dark matter of our Universe.

Alternative hypotheses

{{More|Alternatives to general relativity}}Because dark matter remains to be conclusively identified, many other hypotheses have emerged aiming to explain the observational phenomena that dark matter was conceived to explain. The most common method is to modify general relativity. General relativity is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven. A suitable modification to general relativity can conceivably eliminate the need for dark matter. The most well-known theories of this class are MOND and its relativistic generalization TeVeS,For a review, see: JOURNAL, The failures of the Standard Model of Cosmology require a new paradigm, Pavel Kroupa et al, International Journal of Modern Physics D, December 2012, 21, 4, 10.1142/S0218271812300030, 1301.3907, 2012IJMPD..2130003K, f(R) gravityFor a review, see: JOURNAL, The dark matter problem from f(R) gravity viewpoint, Salvatore Capozziello & Mariafelicia De Laurentis, Annalen der Physik, October 2012, 524, 9–10, 10.1002/andp.201200109, 2012AnP...524..545C, and entropic gravity.NEWS, Delta Institute for Theoretical Physics, New theory of gravity might explain dark matter,weblink Alternative theories abound.JOURNAL, Alternatives to Dark Matter and Dark Energy, Phillip D. Mannheim, Progress in Particle and Nuclear Physics, 56, 2, 10.1016/j.ppnp.2005.08.001, astro-ph/0505266, April 2006, 2006PrPNP..56..340M, JOURNAL, Beyond the Cosmological Standard Model, Austin Joyce et al, Physics Reports, March 2015, 568, 10.1016/j.physrep.2014.12.002, 1407.0059, 2015PhR...568....1J, A problem with alternative hypotheses is that the observational evidence for dark matter comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity.NEWS,weblink Verlinde's new theory of gravity passes first test, December 16, 2016, JOURNAL, First test of Verlinde's theory of Emergent Gravity using Weak Gravitational Lensing measurements, Margot M., Brouwer, etal, Monthly Notices of the Royal Astronomical Society, 466, to appear, 11 December 2016, 10.1093/mnras/stw3192, 1612.03034, 2547–2559, 2017MNRAS.466.2547B, WEB,weblink First test of rival to Einstein's gravity kills off dark matter, 15 December 2016, 20 February 2017, The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter.WEB,weblink Dark Matter vs. Modified Gravity: a Trialogue, Sean M. Carroll, Sean Carroll, 9 May 2012, 14 February 2017,

In philosophy of science

In philosophy of science, dark matter is an example of an auxiliary hypothesis, an ad hoc postulate that is added to a theory in response to observations that falsify it. It has been argued that the dark matter hypothesis is a conventionalist hypothesis, that is, a hypothesis that adds no empirical content and hence is unfalsifiable in the sense defined by Karl Popper.Merritt, David "Cosmology and Convention", Studies In History and Philosophy of Science Part B: Studies In History and Philosophy of Modern Physics, 57(1):41–52, February 2017.

In popular culture

Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the hypothesized properties of dark matter in physics and cosmology.

See also

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Related theories


Experiments
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Dark matter candidates
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Notes

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References

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External links

{{Commons category|Dark matter}} {{Dark matter}}{{Standard model of physics}}{{Cosmology topics}}{{Authority control}}


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