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dark matter, material that is believed to make up more than 90% of the mass of the universe but is not readily visible because it neither emits nor reflects ELECTROMAGNETIC RADIATION, such as light or radio signals. Its existence would explain gravitational anomalies seen in the motion and distribution of galaxies. Dark matter can only be detected indirectly, e.g., through the bending of light rays from distant stars by its gravity. It 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, and brown dwarfs. Other theories hold that it is made of ELEMENTARY PARTICLES that played a key role in the formation of the universe, possibly the low-mass NEUTRINO or theoretical particles called axions and WIMPs [Weakly Interacting Massive Particles]. Attempts by astronomers to identify dark matter that exists in sufficient amounts to explain the structure of the universe appear to have ruled out black holes, brown dwarfs, and other types of ordinary matter as the primary component of dark matter.
Observations of visible matter, the only kind we can see directly, suggest that most of the universe is, in fact, composed of dark matter. This conclusion comes mainly from the belief that something unseen (dark matter) is tugging on visible matter, making it do things the laws of motion say it should not do. All visible bodies, therefore, seem to be careening about in a dense cloud of unseen, unknown masses. These might be dark, Jupiter-sized objects, black holes, and/or some exotic forms of matter. We must choose between the reality of dark matter or admit that something is awry with our laws of gravitation and motion when they are applied on a cosmological scale.
Now, let us examine those four darkmatter items from the recent literature:
D. Lin, a University of California astronomer,
has shown that the Large Magellanic Cloud that orbits around our own galaxy
Way) is being torn apart ("cannibalized") by the powerful gravitational
pull of a dense cloud of dark matter surrounding the Milky Way. This dismemberment
of the Large Magellanic Cloud cannot be explained by the gravitational
forces exerted by the stars in our galaxy that we can see. Lin calculates
that our halo of dark matter is equivalent to 600-800 billion solar masses,
compared to the only 100 billion solar masses of visible matter.
(Flam, Faye; "Spinning in the Dark," Science, 260:1593, 1993. Also: Anonymous; "'Dark Matter' Is Observed 'Cannibalizing' a Galaxy," Baltimore Sun, p. 8A, June 8, 1993.)
The dark matter surrounding a galaxy will, according to the Theory of Relativity, act as a gravitational lens that will deflect light rays passing near it. This dark matter, acting like a telescope, should increase the number of quasars counted in the sky near galactic clusters. Such larger quasar counts are indeed observed, but these increases are much larger than expected. The implication is that there is much more dark matter in the universe than previously thought. (Cowen, Ron; "Quasar Count Poses Dark-Matter Puzzle," Science News, 143:397, 1993.)
Finally, dark matter is forcing scientists to reexamine the Equivalence Principle, which asserts that gravitational mass (as in Newton's Law of Gravitation) is identical to inertial mass (as in Newton's Force = Mass X Acceleration). In terrestrial experiments, the two kinds of mass are equal, but on a cosmological scale, they may not be. There could be an extra, small (10%), longrange, non-gravitational force exerted between massive objects.
(Frieman, Joshua A., and Gradwohl, Ben-Ami; "Dark Matter and the Equivalence Principle," Science, 260:1441, 1993.)
first mention of dark Matter in Usenet:
From: Paul Dietz (dietz@cornell.UUCP)
Subject: What are axions?
Date: 1984-06-17 17:57:34 PST
I've read about certain hypothetical elementary particles called axions. Can some physics guru tell me just what these things are? Here's what I've heard about them: (1) they are electrically neutral, (2) they have a low mass (in the eV-KeV range), and (3) they may be the "dark matter" in our galaxy.
Also, I've heard someone has proposed detecting axions by converting them to photons using a magnetic field with rapid spatial variations. If the density of axions is high enough this could make an excellent energy source -- in effect, total conversion of (axionic) matter to usable radiant energy.