PRINCETON, N.J. (AP) — A pioneering astronomer who helped find powerful evidence of dark matter has died. Vera Rubin was 88.

Allan Rubin said Monday his mother died of natural causes Sunday. He says the Philadelphia native had been living in the Princeton, New Jersey, area.

Rubin found that galaxies don’t quite rotate the way they were predicted, and that lent support to the theory that some other force is at work, namely dark matter.

Dark matter, which hasn’t been directly observed, makes up 27 percent of universe — as opposed to 5 percent of the universe being normal matter. Scientists better understand what dark matter isn’t rather than what it is.

Rubin’s scientific achievements earned her numerous honors, including becoming the second female astronomer to be elected to the National Academy of Sciences.

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