In 1933 Fritz Zwicky suggested that the high speeds of galaxies in the relatively distant Coma Cluster are indicative of some type of invisible material which is pulling on them. In the 1970's Dr. Vera Rubin began to measure the speeds of stars in galaxies using the Doppler shift. She expected stars orbiting at different distances from the centers of spiral galaxies to behave like the planets in our solar system since for these giant star systems the luminous mass that we observe is concentrated towards their centers.  To her amazement the stars near the edge of the great galaxy in Andromeda and many other spiral galaxies are moving at speeds which indicate that these galaxies contain ten times the amount of mass that emits radiation which is visible to us. It was at this point that Dr. Rubin realized that she had discovered compelling evidence to support Zwicky's dark matter hypothesis.