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More about the dark side
In 1933 Fritz Zwicky was studying the motion of galaxies within the Coma Cluster. He found that the motion could only be explained if there was considerably more mass present than what could be observed. Later, in the late 1970s, Vera Rubin was observing the rotational dynamics of the andromeda galaxy. She was attempting to verify theories of how such a galaxy should be spinning. Again, the motion didn’t fit the theory. Again, only having more mass there than could be observed could explain the motion. Thus, we have “dark matter”—a form of mass only observable by its gravitational effects.