dark matter
–noun
a hypothetical form of matter invisible to electromagnetic radiation, postulated to account for gravitational forces observed in the universe.
[Origin: 1985–90]
Dictionary.com Unabridged (v 1.1)
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.
American Heritage Dictionary - Cite This Source
dark matter
n. Physical objects or particles that emit little or no detectable radiation of their own and are postulated to exist because of unexplained gravitational forces observed on other astronomical objects. Dark matter is believed to be part of the missing mass.
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The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2000 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
WordNet - Cite This Source
dark matter
noun
(cosmology) a hypothetical form of matter that is believed to make up 90 percent of the matter in the universe; it is invisible (does not absorb or emit light) and does not collide with atomic particles but exerts gravitational force
WordNet® 2.1, © 2005 Princeton University
The American Heritage Science Dictionary - Cite This Source
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.
Our Living Language : 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 © 2002 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
American Heritage New Dictionary of Cultural Literacy, Third Edition - Cite This Source
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.
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