dark matter

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a hypothetical form of matter invisible to electromagnetic radiation, postulated to account for gravitational forces observed in the universe.
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Origin of dark matter

First recorded in 1985–90
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022


What does dark matter mean?

Dark matter is speculated to be a type of matter that cannot be directly observed. It has an unknown nature, a high mass, and astronomers hypothesize that it makes up much of the universe.

How is dark matter pronounced?

[ dahrk mat-er ]

Where does dark matter come from?

As NASA explains on its website, the dark in dark matter comes from the fact that “it is not in the form of stars and planets that we see.”

In the late 19th century, astronomers noticed dark regions in otherwise dense clusters of stars. In 1904, in an effort to address the problem, Lord Kelvin found that the estimated mass of the Milky Way Galaxy was less than its observable mass, and attributed the difference to stars that were so-called “dark bodies.” Answering Kelvin in 1906, French physicist Henri Poincaré argued there wasn’t any special “matière obscure,” translated as (and the first known use of) dark matter, but rather undetected, non-light-emitting stars. The term dark matter was in use by scientists as an English term by 1922.

As astronomy and astrophysics progressed throughout the 20th century, inconsistencies about the way the universe was assumed to work began to add up. For instance, stars in the outer regions of spiral galaxies moved faster than they should, which suggested these galaxies had far more mass than scientists had accounted for. Astronomers Vera Rubin and Kent Ford, working on this problem, suggested this might be accounted for by dark matter. Throughout the 1980s and 1990s, the existence of dark matter, sometimes characterized as cold and slow-moving, was gradually accepted by many scientists as other pieces of evidence began to build up.

But the story of dark matter is far from over. Scientists don’t exactly know what it consists of, even if it really exists in the form that the term implies, and how to even define it in the first place, making it one of the top, unsolved problems in physics. They do know, however, that dark matter is not galaxy-sized black holes, because we’d be able to detect the light from gravitational lensing (bending of light from a more distant source), and that it’s not antimatter, as antimatter makes tiny explosions upon contact with matter. Nor is it made of dark clouds of normal matter, called baryonic matter, because that would absorb radiation and could be detected.

Some theories suggest a link between dark matter and smaller black holes, dense bodies like brown dwarf stars, extra dimensions, or parallel worlds. Others suggest that it’s just a type of weakly-interacting particle that we can’t detect yet, while still others have proposed a new theory of gravity to account for the issues dark matter suggests.

Some scientists estimate that dark matter makes up about 27% of the universe around us. Although that doesn’t seem like a lot, they also believe that 68% is dark energy, an unknown form of energy (modeled on the name dark matter) that’s causing the universe to expand faster than we expect. If that’s true, it means only about 5% of the universe is made of the matter we understand as “matter.”

Dark matter is now such a well-known term in the sciences that it has come to be used in reference to other things that are unobservable or not completely understood.

Examples of dark matter

“Maps of the cosmic microwave background, the glow of radiation left over from the Big Bang, reveal distortions where light may have been bent by dark matter’s gravity.”
—Devin Powell, “In pursuit of dark matter,” Proceedings of the National Academy of Sciences of the United States of America (August 27, 2013)

“No one’s claiming they’ve actually detected dark matter here, so don’t freak out just yet. But with the hunt for dark matter getting so dire that scientists are now actively looking for ways the Universe makes sense without it, any evidence that validates its existence is a big deal.”
—Fiona MacDonald, “Two Independent Teams Find Hints of Dark Matter in Space Station Data,” Science Alert (May 13, 2017)


This content is not meant to be a formal definition of this term. Rather, it is an informal summary that seeks to provide supplemental information and context important to know or keep in mind about the term’s history, meaning, and usage.

How to use dark matter in a sentence

British Dictionary definitions for dark matter

dark matter

astronomy matter known to make up perhaps 90% of the mass of the universe, but not detectable by its absorption or emission of electromagnetic radiation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for dark matter

dark matter

Matter that emits little or no detectable radiation. Gravitational forces observed on many astronomical objects suggest the significant presence of such matter in the universe, accounting for approximately 23 percent of the total mass and energy of the universe. Its exact nature is not well understood, but it may be largely composed of varieties of subatomic particles that have not yet been discovered, as well as the mass of black holes and of stars too dim to observe. Also called missing mass

A Closer Look

What is the universe made of? We know that galaxies consist of planets, stars, and huge gas and dust clouds-all of these objects are observable by the radiation they give off, such as radio, infrared, optical, ultraviolet, x-ray, or gamma-ray radiation, and all can be observed using various kinds of telescopes. But there are reasons to suspect the existence of far more matter than this, matter that is not directly observable. Evidence for such dark matter comes from observations of certain gravitational effects. For example, astronomers have found that galaxies rotate much faster than they would be expected to rotate based solely on their observable mass-in fact, they should be flying apart. One explanation for this apparent anomaly is to assume that the galaxies have much more mass than we can see, and this invisible mass holds them together gravitationally. Various theories of the composition of this invisible dark matter have been proposed, from exotic yet-to-be discovered particles to planet-sized objects made of ordinary matter that are too small or far away to be detected by present-day instruments. But none of these theories are entirely satisfactory, and the fundamental question of what makes up most of the universe remains unanswered.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for dark matter

dark matter

Unseen matter that may make up more than ninety percent of the universe. As the name implies, dark matter does not interact with light or other electromagnetic radiation, so it cannot be seen directly, but it can be detected by measuring its gravitational effects. It is believed that dark matter was instrumental in forming galaxies early in the Big Bang.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.