Dark matter | Definition, Discovery, Distribution, & Facts | Britannica (2024)

astronomy

Print

verifiedCite

While every effort has been made to follow citation style rules, there may be some discrepancies.Please refer to the appropriate style manual or other sources if you have any questions.

Select Citation Style

Feedback

Thank you for your feedback

Our editors will review what you’ve submitted and determine whether to revise the article.

Also known as: hidden mass, missing mass, nonluminous matter

Written by

Adam Riess Adam Riess is Thomas J. Barber Professor in Space Studies, Department of Astronomy and Physics, Johns Hopkins University and Distinguished Astronomer, Space Telescope Science Institute. He shared the 2011...

Adam Riess

Fact-checked by

The Editors of Encyclopaedia Britannica Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.

The Editors of Encyclopaedia Britannica

Last Updated: Article History

Key People:
Ken Freeman
Adam Riess
Vera Rubin
Related Topics:
universe
On the Web:
CORE - Dark Matter and Dark Energy (Nov. 08, 2024)

See all related content

Top Questions

What is dark matter?

Dark matter is a component of the universe whose presence is discerned from its gravitational attraction rather than its luminosity.

What percentage of the matter-energy composition of the universe is made of dark matter?

Dark matter makes up 30.1 percent of the matter-energy composition of the universe. The rest is dark energy (69.4 percent) and “ordinary” visible matter (0.5 percent).

Who first inferred the existence of dark matter?

Dark matter’s existence was first inferred by Swiss American astronomer Fritz Zwicky, who in 1933 discovered that the mass of all the stars in the Coma cluster of galaxies provided only about 1 percent of the mass needed to keep the galaxies from escaping the cluster’s gravitational pull.

What are the types of dark matter?

Two varieties of dark matter have been found to exist. The first variety is about 4.5 percent of the universe and made of the familiar baryons (i.e., protons, neutrons, and atomic nuclei), which also make up the luminous stars and galaxies. The dark matter comprising the other 26.1 percent is in an unfamiliar, nonbaryonic form that is relatively “cold,” or “nonrelativisitic.”

News

Can Axions Save the Universe? Oct. 31, 2024, 11:52 PM ET (New York Times)

dark matter, a component of the universe whose presence is discerned from its gravitational attraction rather than its luminosity. Dark matter makes up 30.1 percent of the matter-energy composition of the universe; the rest is dark energy (69.4 percent) and “ordinary” visible matter (0.5 percent).

Originally known as the “missing mass,” dark matter’s existence was first inferred by Swiss American astronomer Fritz Zwicky, who in 1933 discovered that the mass of all the stars in the Coma cluster of galaxies provided only about 1 percent of the mass needed to keep the galaxies from escaping the cluster’s gravitational pull. The reality of this missing mass remained in question for decades, until the 1970s when American astronomers Vera Rubin and W. Kent Ford confirmed its existence by the observation of a similar phenomenon: the mass of the stars visible within a typical galaxy is only about 10 percent of that required to keep those stars orbiting the galaxy’s centre. In general, the speed with which stars orbit the centre of their galaxy is independent of their separation from the centre; indeed, orbital velocity is either constant or increases slightly with distance rather than dropping off as expected. To account for this, the mass of the galaxy within the orbit of the stars must increase linearly with the distance of the stars from the galaxy’s centre. However, no light is seen from this inner mass—hence the name “dark matter.”

1 of 2
1 of 2
2 of 2

Since the confirmation of dark matter’s existence, a preponderance of dark matter in galaxies and clusters of galaxies has been discerned through the phenomenon of gravitational lensing—matter acting as a lens by bending space and distorting the passage of background light. The presence of this missing matter in the centres of galaxies and clusters of galaxies has also been inferred from the motion and heat of gas that gives rise to observed X-rays. For example, the Chandra X-ray Observatory has observed in the Bullet cluster, which consists of two merging galaxy clusters, that the hot gas (ordinary visible matter) is slowed by the drag effect of one cluster passing through the other. The mass of the clusters, however, is not affected, indicating that most of the mass consists of dark matter.

Britannica QuizAll About Astronomy

Matter is 30.6 percent of the universe’s matter-energy composition. Only 0.5 percent is in the mass of stars and 0.03 percent of that matter is in the form of elements heavier than hydrogen. The rest is dark matter. Two varieties of dark matter have been found to exist. The first variety is about 4.5 percent of the universe and is made of the familiar baryons (i.e., protons, neutrons, and atomic nuclei), which also make up the luminous stars and galaxies. Most of this baryonic dark matter is expected to exist in the form of gas in and between the galaxies. This baryonic, or ordinary, component of dark matter has been determined by measuring the abundance of elements heavier than hydrogen that were created in the first few minutes after the big bang occurred 13.8 billion years ago.

The dark matter that comprises the other 26.1 percent of the universe’s matter is in an unfamiliar, nonbaryonic form. The rate at which galaxies and large structures composed of galaxies coalesced from density fluctuations in the early universe indicates that the nonbaryonic dark matter is relatively “cold,” or “nonrelativisitic,” meaning that the backbones of galaxies and clusters of galaxies are made of heavy, slow-moving particles. The absence of light from these particles also indicates that they are electromagnetically neutral. These properties give rise to the particles’ common name, weakly interacting massive particles (WIMPs). The precise nature of these particles is not currently known, and they are not predicted by the standard model of particle physics. However, a number of possible extensions to the standard model such as supersymmetric theories predict hypothetical elementary particles such as axions or neutralinos that may be the undetected WIMPs.

Extraordinary efforts are under way to detect and measure the properties of these unseen WIMPs, either by witnessing their impact in a laboratory detector or by observing their annihilations after they collide with each other. There is also some expectation that their presence and mass may be inferred from experiments at new particle accelerators such as the Large Hadron Collider.

As an alternative to dark matter, modifications to gravity have been proposed to explain the apparent presence of “missing matter.” These modifications suggest that the attractive force exerted by ordinary matter may be enhanced in conditions that occur only on galactic scales. However, most of the proposals are unsatisfactory on theoretical grounds as they provide little or no explanation for the modification of gravity. These theories are also unable to explain the observations of dark matter physically separated from ordinary matter in the Bullet cluster. This separation demonstrates that dark matter is a physical reality and is distinguishable from ordinary matter.

Dark matter | Definition, Discovery, Distribution, & Facts | Britannica (6)

Get Unlimited Access

Try Britannica Premium for free and discover more.

Subscribe

Adam Riess

Dark matter | Definition, Discovery, Distribution, & Facts | Britannica (2024)
Top Articles
Latest Posts
Recommended Articles
Article information

Author: Virgilio Hermann JD

Last Updated:

Views: 6504

Rating: 4 / 5 (61 voted)

Reviews: 84% of readers found this page helpful

Author information

Name: Virgilio Hermann JD

Birthday: 1997-12-21

Address: 6946 Schoen Cove, Sipesshire, MO 55944

Phone: +3763365785260

Job: Accounting Engineer

Hobby: Web surfing, Rafting, Dowsing, Stand-up comedy, Ghost hunting, Swimming, Amateur radio

Introduction: My name is Virgilio Hermann JD, I am a fine, gifted, beautiful, encouraging, kind, talented, zealous person who loves writing and wants to share my knowledge and understanding with you.