The Hunt for Dark Matter in Galaxies

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Science  12 Dec 2003:
Vol. 302, Issue 5652, pp. 1902-1903
DOI: 10.1126/science.1091466

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Most galaxies are enveloped in huge dark halos that contain 10 to 20 times as much mass as the luminous stars and cold gas in the galaxies. Simulations of structure formation in the universe are not entirely consistent with the properties of dark halos inferred from observation. In his Perspective, Freeman discusses recent results about dark matter obtained by measuring the "gravitational lensing" of distant objects by intervening galaxies. Lensing surveys indicate that dark halos contribute about 11% of the mass of the universe, compared with about 0.4% in stars and cold gas. Intergalactic dark matter and gas probably make up another 16%, and the remaining 73% is in the form of a poorly understood "dark energy."