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"Material whose presence can be inferred from its effects on the motions of stars and galaxies, but which cannot be seen directly because it emits little or no radiation; also known as missing mass. It is thought that at least 90% of the mass in the Universe resides in some form of dark matter. Evidence for dark matter in spiral galaxies is provided by their *rotation curves. The existence of dark matter in rich clusters of galaxies can be deduced from the motions of the member galaxies [see viral theorem]. A significant part of this dark matter may be in the form of low-mass stars or Jupiter-mass objects; such normal matter is described as baryonic. Dark matter may also exist in the space between galaxies, and could raise the average density of the Universe to the *critical density required to reverse the current expansion. If the *Big Bang theory is correct, then a large proportion of dark matter must be in non-bayonic form, perhaps *axions, *photinos, or massive neutrinos, left over from the early stages of the Big Bang."
"A cloud of interstellar gas and dust that absorbs light from behind, so that it appears black against a brighter backround. The absorbed light heats the dust particles, which then reradiate some of the energy as infrared radiation. Some of the backround light is not absorbed but is scattered, or re-directed. The *Horsehead Nebula in Orion is a well-known dark nebula; another is *coalsack in the southern Milky Way."
COLD DARK MATTER (CDM)
"A particular type of non-baryonic matter that, according to some theories, is created in the early stages of the Big Bang and survives in the present time in sufficient numbers to contribute significantly to the present density of the Universe. The term 'cold' signifies that these particles move at speeds much less than that of light, usually because they are heavy. There are many possible canidates for cold dark matter, such as *axions, *photinos, and primorddial (low-mass) black holes. Cold dark matter has, until recently, appeared to help solve the problem of *galaxy formation and *large-scale structure in the Universe. More recent observations, however, suggest that the simplest versions of this picture are not correct"(?)
HOT DARK MATTER
A particular type of *non-baryonic particle that, according to some theories, is created in the early stages of the Big Bang and survives to the present time in sufficent numbers to contribute significantly to the present density of the Universe. The term 'hot signifies that these particles are fast-moving (close to the speed of light), usually because they are of low mass. The favoured candidate for such a particle is a neutrino with a *rest mass of around 10 eV, which is 1/500 000 of the mass of the electron.