Assertion: matter's mass | /r/HasProperty | only about 10 percent of that
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text: The reality of this missing mass remained in question for decades, until the 1970s when American astronomers Vera Rubin and W. Kent Ford confirmed its existence by the observation of a similar phenomenon: the mass of the stars visible within a typical galaxy is only about 10 percent of that required to keep those stars orbiting the galaxy’s centre.