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Galactic dark matter.

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Galactic dark matter (GDM) is matter which appears to constitute some 90% of the total mass of typical galaxies while producing no detectable radiation or absorption.

Strong evidence for the existence of GDM comes from galactic rotation curves derived from the variation in red-shift across a galaxy, interpreted as a velocity distribution. The resulting curves systematically show rotational velocities which rise rapidly from the centre, then remain almost constant to the outermost discernable regions of the galaxy. Provided only that the galaxy is gravitationally bound, this behaviour is not consistent with the visible distribution of matter, for which one would expect velocity to rise across the dense central bulge, but then to fall across the much more tenuous disk. Instead, the evidence of rotation curves strongly supports the view that typical galaxies are imbedded in a "halo" extending far beyond the visible galaxy and comprising some ten times as much mass.

Supporting evidence comes from the relative motions in clusters of galaxies, particularly binary systems.

It is perhaps worth emphasizing that the existence of galactic dark matter is observationally determined, and does not depend on the validity of the "Big Bang" scenario (with or without "Inflation") nor on the value of the Hubble parameter - though these can have some bearing on the relative plausibility of various candidates for the GDM. Also, though the strong evidence for a dominant cosmological constant, "dark energy" or "quintessence" (see e.g. D. N. Spergel et al., astro-ph/0302209; Astrophys. J. Suppl. 148 (2003) 175-194) would reduce the amount of dark matter needed on the largest scales, this has no effect on the smaller scales of galaxies and galactic clusters. Observations suggest that more than 80% of the dark matter on these scales may be non-baryonic, and thus could consist of relic stable elementary particles.