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dark matter (hidden mass)
subject			fact

dark matter	is a part of dark halo
	has synonym hidden mass
	has definition Material astronomers cannot see but whose presence they believe in either because they detect its gravitational influence or because certain theories predict its existence. For example, astronomers believe that the outer part of the Galaxy harbors dark matter, because they notice its gravitational influence on the stars they can see; and inflationary cosmologists believe that the universe is full of dark matter, because inflation predicts that the universe has a large density.
	has definition Any form of matter which exists in the Universe in a non-luminous form.
	has definition Matter whose existence is inferred on the basis of dynamical studies - e.g., the orbits of stars, in galaxies - but which does not show up as bright objects such as stars and nebulae. Its composition is unknown: It might consist of subatomic particles, or of dim dwarf stars or black holes, or a combination of various sorts of objects.
	has definition Matter that is invisible to us because it emits little or no light. As much as 90%-99% of the mass of the universe may be dark.
	has definition Matter that is detected only by its gravitational pull on visible matter. At least 90%, and possibly 99% of the matter in the universe is dark. The composition is unknown; it might consist of very low mass stars or supermassive black holes, but big-bang nucleosynthesis calculations limit the amount of such baryonic matter to a small fraction of the critical mass density. If the mass density is critical, as predicted by the simplest versions of inflation, then the bulk of the dark matter must be a gas of weakly interacting non-baryonic particles, sometimes called WIMPS (Weakly Interacting Massive Particles). Various extensions of the standard model of particle physics suggest specific candidates for the WIMPs.
	has definition Matter in the universe that we detect by its gravitational influences, yet do not see. Dark matter that has small random speed and is easily concentrated by gravity is called cold dark matter. Dark matter that has large random speed and is thus able to resist gravitational clumping is called hot dark matter. Recent models to explain the observed pattern of galaxy clustering can be characterized, in part, as to whether they invoke hot dark matter or cold dark matter. However, since we do not know what the dark matter is, we do not have any direct evidence of whether it is cold or hot.
	has definition Matter whose presence is inferred from dynamical measurements but which has no optical counterpart. The luminous regions of galaxies have mass-luminosity ratios of about 10. However, the mass-luminosity ratio in the outer halos of many spiral galaxies is 100 or more; one sees the brightness fall off with distance from the center of the galaxy but considerable mass is present. A similar situation prevails in galaxy clusters, where nonluminous matter must provide most of the self-gravitational attraction that holds the clusters together. The missing mass is not really missing; it is present but invisible (at least to current detectors). It is generally believed to consist either of the remnants of massive stars or of planetary-sized objects comparable in mass to Jupiter.
	is a kind of hypothetical particle
hypothetical particle	has acceptance status hypothetical
particle	obeys uncertainty principle
	has frequency inversely proportional to the wavelength
	has wavelength inversely proportional to its momentum
	has charge
	has mass
physical object	has location or center of gravity
	has angular momentum
	has velocity
	has momentum
	has temperature
	has volume
	has extent
	has material

Kinds of dark matter :

• cold dark matter (1 kind, 9 facts) (CDM) - Hypothetical subatomic particles that move slowly compared with the speed of light., Any dark matter candidate which was non-relativistic at the point of decoupling., A model of structure formation in which an exotic particle whose energy is low at the time it decouples from other matter is responsible for structure formation.
• hot dark matter (2 facts) - Any form of dark matter which was relativistic at its point of decoupling.
• massive compact halo object (21 kinds, 95 facts) (MACHO) - These are black holes, neutron stars and brown dwarfs, none of which are luminous and all of which are postulated to exist in the halos of galaxies. They are a form of dark matter.

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