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We can tell how much matter is in the universe by the motions of the stars. In the1920s, physicists attempting to do so discovered a discrepancy and concluded that there must be more matter in the universe than is detectable. How can this be?
In 1933, Swiss astronomer Fritz Zwicky, while observing the motion of galaxies in the Coma Cluster, began wondering what kept them together. There wasn’t enough mass to keep the galaxies from flying apart. Zwicky proposed that some kind of dark matter provided cohesion. But since he had no evidence, his theory was quickly dismissed.
Then, in 1968, astronomer Vera Rubin made a similar discovery. She was studying the Andromeda Galaxy at Kitt Peak Observatory in the mountains of southern Arizona when she came across something that puzzled her. Rubin was examining Andromeda’s rotation curve or the speed at which the stars around the center rotate, and realized that the stars on the outer edges moved at the exact same rate as those at the interior, violating Newton’s laws of motion. This meant there was more matter in the galaxy than was detectable. Her punch card readouts are today considered the first evidence of the existence of dark matter.
Many other galaxies were studied throughout the ’70s. In each case, the same phenomenon was observed. Today, dark matter is thought to comprise up to 27% of the universe. “Normal” or baryonic matter makes up just 5%. That’s the stuff we can detect. Dark energy, which we can’t detect either, makes up 68%.
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