The Missing Mass

In the 1930s, Fritz Zwicky found galaxies in the Coma Cluster moving too fast for visible mass to hold them together. He concluded unseen "dark matter" must supply the extra gravitational glue keeping them from flying apart.

In the 1970s, Vera Rubin confirmed the problem: stars at galaxy edges orbit just as fast as those near the center. These flat rotation curves became major evidence that galaxies sit inside massive halos of invisible matter.

They did not identify the particle. They showed that visible matter and standard gravity, by themselves, could not explain the motion.