In the early 1970s, astronomer Vera Rubin and Kent Ford at the Carnegie Institution for Science attached the Image Tube Spectrograph to several large telescopes to analyze distant spiral galaxies. This state-of-the-art instrument allowed telescopes to observe objects that were many times fainter than those that had been previously studied. What they found would change our understanding of the universe: The galaxies’ outer arms were rotating at velocities that should have made their stars fly away—but didn’t. The only explanation, Rubin decided, was that the galaxies contained far more mass than we could see. It was the strongest evidence yet for the existence of dark matter, now believed to make up 26.8 percent of all the stuff that exists.
Rubin and DTM collaborators Ford, Norbert Thonnard, and John Graham were among the first astronomers to examine the systemic velocities of galaxies to see if there are large-scale motions of galaxies, superposed on the general expansion of the universe. Their early work, and more recent work by others, suggests that such motions exist.