Vera Rubin's galaxy rotation curves reveal dark matter

Using a sensitive spectrograph built by Kent Ford, Vera Rubin measured how fast stars and gas orbit within the Andromeda galaxy. Contrary to expectation, the speed did not fall off toward the edge; stars far from the centre moved just as fast as those near it. Gravity from the visible matter could not hold them, so each galaxy had to be embedded in a massive halo of unseen material. Rubin's results, extended to dozens of galaxies by the late 1970s, turned dark matter from a curiosity into one of the central problems of physics.

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