Vera Rubin analyzed the blue stars in our galaxy, she found that the stars rotated around the galaxy at the same rate, independent of their distance from the galactic center (which is called a flat rotation curve), thereby violating the precepts of Newtonian mechanics. In fact, she found that the Milky Way galaxy was rotating so fast that, by rights, it should fly apart. But the galaxy has been quite stable for about 10 billion years; it was a mystery why the rotation curve was flat. To keep the galaxy from disintegrating, it had to be ten times heavier than scientists currently imagined.
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