Vera Rubin found strong evidence for dark matter by studying the rotation of galaxies in the 1970s.
Rubin and collaborator Kent Ford measured how quickly stars moved around the centers of spiral galaxies. They found that stars far from galactic centers were orbiting much faster than expected from the visible matter alone.
The result suggested that galaxies are embedded in large, invisible halos of matter. This unseen material became known as dark matter and is now a central concept in cosmology. Rubin’s observations confirmed and greatly strengthened earlier theoretical suggestions that additional mass existed.
Rubin is often described as the discoverer of dark matter, but that wording simplifies a longer history. Her rotation-curve observations provided especially compelling evidence for dark matter’s presence; they did not directly detect the substance or identify what it is made of.