© NASA, JPL-Caltech, SDSS, Leigh Jenkins, Ann Hornschemeier (Goddard Space Flight Center) et al.
This image combines data from the Spitzer Space Telescope with the Sloan Digital Sky Survey to show many of the thousands of galaxies in the Coma cluster. By measuring the velocities of these galaxies, Fritz Zwicky realized that there is more to the Coma cluster than meets the eye. Galaxies toward the edge of the cluster were moving far too fast, if their motions were to be explained by the gravitational influence of the other galaxies in the cluster. Zwicky took this as evidence that the cluster must contain a great deal of matter that he couldn't see with his telescope—the first evidence for dark matter. (Unit: 10)