dark matter, material that is believed to make up (along with dark energy dark energy, repulsive force that opposes the self-attraction of matter (see gravitation ) and causes the expansion of the universe to accelerate. The search for dark energy was triggered by the discovery (1998) in images from the Hubble Space Telescope of a distant
..... Click the link for more information. ) more than 90% of the mass of the universe but is not readily visible because it neither emits nor reflects electromagnetic radiation electromagnetic radiation, energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an
..... Click the link for more information. , such as light or radio signals. Its existence would explain gravitational anomalies seen in the motion and distribution of galaxies. Dark matter can be detected only indirectly, e.g., through the bending of light rays from distant stars by its gravity.

Dark matter may consist of dust, planets, intergalactic gas formed of ordinary matter, or of MACHOs [Massive Astrophysical Compact Halo Objects], nonluminous bodies such as burned-out stars, black holes black hole, in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping.
..... Click the link for more information. , and brown dwarfs brown dwarf, in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do.
..... Click the link for more information. ; these are the so-called hot dark matter and would be dispersed uniformly throughout the universe. The discovery in 2001 of a large concentration of white dwarf white dwarf, in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation ). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be
..... Click the link for more information. stars in the halo surrounding the Milky Way Milky Way, the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
..... Click the link for more information. indicates that these burned-out stars could represent as much as a third of the dark matter in the universe.

Other theories hold that it is made of elementary particles elementary particles, the most basic physical constituents of the universe.

Basic Constituents of Matter

Molecules are built up from the atom , which is the basic unit of any chemical element .
..... Click the link for more information. that played a key role in the formation of the universe, possibly the low-mass neutrino neutrino (n

trē`nō) [Ital.
..... Click the link for more information. or theoretical particles called axions and WIMPs [Weakly Interacting Massive Particles]; these are the so-called cold dark matter and would be found in clumps throughout the universe. In 1996 a Japanese team at the Univ. of Tokyo led by Yasushi Ikebe reported on dark-matter clumping in the galactic cluster Fornax. Clumps were found in two distinct regions: around a massive galaxy in the center of the cluster and, in larger amounts, around the entire cluster. This suggests that the slower, cold dark matter might form the smaller clumps associated with the galaxy while the faster, hot dark matter might form the larger clumps associated with the galactic cluster.

Computer simulations of the formation of the universe favored the cold dark matter but tended to predict the formation of too many dwarf galaxies when compared to the observed universe. This led to the postulation of warm dark matter, which resolved the simulation problems. Unlike cold dark matter, which has mass but virtually no velocity or temperature, or hot dark matter, which has mass and is highly energetic, warm dark matter has mass and a low temperature corresponding to an extremely low velocity.

See also interstellar matter interstellar matter, matter in a galaxy between the stars, known also as the interstellar medium.

Distribution of Interstellar Matter

Compared to the size of an entire galaxy, stars are virtually points, so that the region occupied by the
..... Click the link for more information. .

Bibliography

See R. Morris, Cosmic Questions: Galactic Halos, Cold Dark Matter and the End of Time (1995); T. Van Flandern, Dark Matter, Missing Planets, and New Comets (2d ed. 1998); M. Hawkins, Hunting Down the Universe: The Missing Mass, Primordial Black Holes and Other Dark Matters (1999).

dark matter

Nonluminous matter not directly detectable by astronomers, hypothesized to exist because the mass of the visible matter in the universe cannot account for observed gravitational effects. Long believed to exist in large quantities, it enters into many theories of the origin of the universe and its present large-scale structure and into models of gravitation and other fundamental forces (see fundamental interaction) between particles. Numerous candidates for dark matter have been proposed over the years, but none has yet been confirmed.