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Dark matter

In astronomy, dark matter is a hypothetical form of matter that appears not to interact with light or the electromagnetic field. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be seen. Such effects occur in the context of formation and evolution of galaxies,[1] gravitational lensing,[2] the observable universe's current structure, mass position in galactic collisions,[3] the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.

For other uses, see Dark Matter (disambiguation). Not to be confused with antimatter or dark energy.

In the standard lambda-CDM model of cosmology, the mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy.[4][5][6][7] Thus, dark matter constitutes 85%[a] of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content.[8][9][10][11]


Dark matter is not known to interact with ordinary baryonic matter and radiation except through gravity,[b] making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered subatomic particle,[c] such as weakly interacting massive particles (WIMPs) or axions.[12] The other main possibility is that dark matter is composed of primordial black holes.[13][14][15]


Dark matter is classified as "cold", "warm", or "hot" according to its velocity (more precisely, its free streaming length). Recent models have favored a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles.


Although the astrophysics community generally accepts dark matter's existence,[16] a minority of astrophysicists, intrigued by specific observations that are not well-explained by ordinary dark matter, argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics, tensor–vector–scalar gravity, or entropic gravity. So far none of the proposed modified gravity theories can successfully describe every piece of observational evidence at the same time, suggesting that even if gravity has to be modified, some form of dark matter will still be required.[17]

History[edit]

Early history[edit]

The hypothesis of dark matter has an elaborate history.[18] In the appendices of the book Baltimore lectures on molecular dynamics and the wave theory of light where the main text was based on a series of lectures given in 1884,[19] Lord Kelvin discussed the potential number of stars around the Sun from the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20 to 100 million years old. He posed what would happen if there were a thousand million stars within 1 kilo-parsec of the Sun (at which distance their parallax would be 1 milli-arcsec). Lord Kelvin concluded:

From the scatter in radial velocities of the galaxies within clusters

From 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.

X-rays

(usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

Gravitational lensing

Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.

The theory of 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.[104][105] 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.[83]

Big Bang nucleosynthesis

Astronomical searches for in the Milky Way found at most only 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.[106][107][108][109][110][111]

gravitational microlensing

Detailed analysis of the small irregularities (anisotropies) in the .[112] 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.

cosmic microwave background

Dark matter serves as a plot device in the episode "Soft Light."[192]

X-Files

A dark-matter-inspired substance known as "Dust" features prominently in 's His Dark Materials trilogy.[193]

Philip Pullman

Beings made of dark matter are antagonists in 's Xeelee Sequence.[194]

Stephen Baxter

Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,[190] and dark matter itself has been referred to as "the stuff of science fiction".[191]


Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example:


More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.[195]

; McGaugh, Stacy S. (August 2018). "Is dark matter real?". Scientific American. Vol. 319, no. 2. pp. 36–43.

Hossenfelder, Sabine

(July/August 2023) "The Dark Universe Comes into Focus" Scientific American, vol. 329, no. 1 , p. 7-8.

Weiss, Rainer

Cirelli, Marco; Strumia, Alessandro; Zupan, Jure (2024). "Dark Matter". :2406.01705 [hep-ph].

arXiv

at Curlie

Dark matter

Tremaine, Scott. (Video). IAS.

Lecture on dark matter

Gray, Meghan; Merrifield, Mike; Copeland, Ed (2010). (ed.). "Dark Matter". Sixty Symbols. University of Nottingham.

Haran, Brady