Dark matter

n. (Astronomy) non luminous matter presumed to exist in space

In astrophysics and cosmology, dark matter is hypothetical matter of unknown composition 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 galaxy-sized as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe.  Fritz Zwicky used it for the first time to declare the observed phenomena consistent with dark matter observations as the rotational speeds of galaxies and orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component".
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Dark•Matter is a science fiction/conspiracy theory campaign setting that was originally published in 1999 by Wizards of the Coast for the Alternity role-playing game. It was written by Wolfgang Baur and Monte Cook. It was later converted to d20 Modern rules and published as a stand-alone book in 2006.
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Noun
1. (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
(hypernym) substance, matter
(part-meronym) weakly interacting massive particle, WIMP
(classification) cosmology, cosmogony, cosmogeny

Hypothetical matter which is postulated to exist in the universe in vast quantities. It is difficult to detect because it is either non luminous or possessed of a very low luminosity . Dark matter is thought to exist because of its gravitational effects in the universe; specifically within galaxies and clusters of galaxies. Within a spiral galaxy, the stars move as if large quantities of matter, which cannot be seen, exist around the disc of the galaxy. Within clusters of galaxies, the individual galaxies move as if ten times as much matter exists than can be seen in the stars and the emission nebulae. Dark matter may come in two principle types: the first is baryonic matter. This is matter such as makes up the luminous portions of the universe. The atoms of the familiar chemical elements are simply bound into intrinsically low luminosity objects such as planets , brown dwarfs and black holes . If the matter is not in this form, then the second type is dark matter which is thought to exist in the form of the exotic particles. These are predicted by certain Grand Unified Theories . Two sub-classifications of exotic dark matter exist; cold dark matter and hot dark matter. Hot dark matter is composed of particles such as neutrinos. They are called hot because they travel at (or very close to) the speed of light. Neutrinos are a prime candidate, although there is still uncertainty over whether they have any mass. If neutrinos do not have mass, they cannot provide the gravity necessary to act as the dark matter. Cold dark matter is composed of weakly interacting massive particles (WIMPs). These particles are possessed of relatively large masses, travel relatively slowly and interact only weakly with 'normal' baryonic material. Hence, they are difficult to detect. Although many experiments have been undertaken to detect neutrino masses and WIMPs, no conclusive evidence has yet been found and the search for what the constituents are in dark matter goes on.