from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun Matter that emits little or no detectable radiation of its own, postulated to account for observed gravitational forces that affect astronomical objects but have no observable sources. Dark matter is thought to be part of the missing mass.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun astronomy Particles of matter that cannot be detected by their radiation but whose presence is inferred from gravitational effects.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun (cosmology) a hypothetical form of matter that is believed to make up 90 percent of the universe; it is invisible (does not absorb or emit light) and does not collide with atomic particles but exerts gravitational force


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  • In astrophysics and cosmology, dark matter is a hypothetical form of matter that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe. (Wikipedia)

    May 20, 2008