dark matter

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dark matter,

material that is believed to make up nearly 27% of the mass of the universe but is not readily visible because it neither emits nor reflects electromagnetic radiationelectromagnetic 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 electric field.
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, 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 holesblack 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.
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, and brown dwarfsbrown 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. Also called "failed stars," brown dwarfs form in the same way as true stars (by the contraction of a swirling
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; 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 dwarfwhite 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 easily
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 stars in the halo surrounding the Milky WayMilky 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.
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 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 particleselementary 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. The atom in turn is made from the proton, neutron, and electron.
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 that played a key role in the formation of the universe, possibly the low-mass neutrinoneutrino
[Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy
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 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 dark energydark 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 supernova
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; interstellar matterinterstellar 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 interstellar matter
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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

Matter that probably comprises 75% (or more) of the mass of the Universe but is undetectable except by its gravitational effects. Dark matter was first suspected in clusters of galaxies when the galaxies were found to move with too high a speed to be retained in the cluster by their gravitational influence on each other. The term missing mass was coined for the necessary invisible matter in the cluster, amounting to 10 to 100 times the amount of visible matter in the galaxies and in the intergalactic medium. In the 1970s investigations of the rotation of spiral galaxies indicated that these galaxies have a dark halo containing some 10 times as much matter as the visible galaxy. Simultaneously statistical analyses of the motions of galaxies on a larger scale than clusters suggested that the Universe in general has a ratio of perhaps 4:1 of dark matter to baryonic matter (except within galaxies, where ordinary matter dominates).

The nature of the dark matter is highly elusive, and there is no certainty even that the dark haloes of spiral galaxies consist of the same matter as the missing mass of clusters and the Universe at large. A proportion may consist of objects composed of normal matter, but according to standard Big-Bang models the whole of the missing mass cannot consist of matter containing baryons (protons and neutrons) since that amount of baryonic matter would have produced a higher abundance of helium than is observed. The bulk of the dark matter does not therefore consist of dim stars or smaller chunks of solid matter, nor of black holes that have formed from stars.

If the dark matter candidate was moving at velocities comparable to the speed of light as it decoupled from the early Universe, it would subsequently smear out all early fluctuations except on the very largest scales (see microwave background radiation). This candidate, known as hot dark matter, thus predicts that superclusters of galaxies were the first structures to form, fragmenting later to form bound clusters and galaxies in a top-down scenario. This theory is no longer in favor because it has problems with the formation of galaxies at high redshifts, and is in contradiction with the anisotropies detected in the microwave background by the satellite COBE.

Alternatively, cold dark matter (CDM) particles are created and have low velocity dispersions now and would have become trapped in baryonic gravitational potentials of galactic size when they decoupled. Clusters and superclusters would thus form later in the life of the Universe due to the mutual gravitational attraction of the galaxies. This bottom-up scenario is known as hierarchical clustering. Computer simulations of galaxy clustering, the observed galaxy correlation function, and recent X-ray observations of cluster evolution support the concept of cold dark matter. While consistent with the COBE observations, the CDM model needs further refinements to produce the observed large-scale structure of the Universe from the original spectrum of microwave-background fluctuations.

Candidates for cold dark matter are exotic elementary particles, including closed cosmic strings, WIMPs, axions, and supersymmetry particles, such as neutralinos, photinos, and gravitinos. Hot dark matter candidates include neutrinos with nonzero but small mass.

See also galaxies, formation and evolution.

dark matter

[′därk ‚mad·ər]
Matter that is postulated to exist to explain the rotational motion of the Milky Way Galaxy and other galaxies, to explain the motions of galaxies in clusters, and, in certain cosmological theories, to achieve the critical density of matter in the universe that is just sufficient to close the universe. Also known as missing mass.
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