Dark Matter Found in a "Typical" Cluster of Galaxies

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January 4, 1993: Astronomer John S. Mulchaey, of the Space Telescope Science Institute (STSCI) and fellow team members announced today the discovery of a huge concentration of mysterious "dark matter" with the ROSAT X-ray observatory satellite. ROSAT is an acronym for Roentgen Satellite, a joint project of Germany, NASA, and the U.K.)