Why is dark matter needed to explain galaxies and clusters?

Astronomers found that stars in the outer regions of galaxies orbit too fast to be held together by visible matter alone. Fritz Zwicky first noted missing mass in galaxy clusters in the 1930s, and Vera Rubin’s work in the 1970s strengthened the case from galaxy rotation curves. Dark matter does not emit or absorb light, so it is invisible except through gravity. It appears to make up about 85 percent of the matter in the universe. Although its exact nature remains unknown, it is a cornerstone of modern cosmology.