In the late 1970s, Vera Rubin and Kent Ford of the Carnegie Institution of Washington stared, confused, at the punch-card readouts from their observations of the Andromeda Galaxy. The vast spiral seemed to be rotating all wrong. The stuff at the edges was moving just as fast as the stuff near the center, apparently violating Newton’s Laws of Motion (which also govern how the planets move around our Sun). While the explanation for that strange behavior didn’t become clear to Rubin until two years later, these printouts represented the first direct evidence of dark matter.

How Vera Rubin discovered dark matter | Astronomy.com

hmmm