In the 1930s, astronomer Fritz Zwicky (1898–1974) noticed that, in the Coma cluster of galaxies, many of the individual galaxies were moving around so fast that there had to be a tremendous amount of gravitational pull toward the center of the cluster; otherwise, the galaxies would literally fling themselves out of the cluster. The amount of matter that needed to exist in the cluster to produce that much gravity far exceeded the amount of matter observed in all the galaxies in the cluster put together. This extra matter became known as “dark matter.”

In 1970 astronomer Vera C. Rubin (1928–) and physicist W. Kent Ford (1931–) showed that stars in the Andromeda Galaxy were moving so fast that for the stars to stay in the galaxy there had to be a tremendous amount of matter surrounding and enveloping the entire galaxy like a giant cocoon. Because this matter is not visible to telescopes by the light it emits, but rather only by the gravity it exerts, this, too, is an example of evidence for dark matter.

After decades of further study, dark matter has now been confirmed as an important constituent of matter around galaxies, in clusters of galaxies, and throughout the universe as a whole. According to the latest measurements, about 85 percent of the matter in the universe is dark matter.