In 1965 Vera Rubin became the first woman formally permitted to observe at Palomar, a mountain whose administrators explained that her time there would have to be short because the building had no ladies' room. She cut a small skirt out of paper, taped it onto the stick figure on the men's room door, announced that the facility was now compliant, and went to work. She then spent fifteen years producing one of the most durable measurements in the history of astronomy.

The measurement is simple enough to explain at a bus stop. In the solar system the outer planets crawl and the inner ones sprint, because nearly all the mass sits in the middle and gravity thins with distance. In a spiral galaxy, Rubin found, the outer stars move just as fast as the inner ones. The curve that should have sloped downward stayed flat. It stayed flat in Andromeda, then in twenty-one galaxies, then in sixty, then in everything anyone pointed a spectrograph at. Something holds those galaxies together, and it is not the part that shines.

That much is settled, and this book does not dispute a single one of her numbers. What it disputes is the sentence that got printed on the mug afterward. Rubin discovered a discrepancy. Within a decade the discrepancy had acquired a name, and within two the name had quietly acquired a mass, a density, an abundance, a preferred particle, a set of detectors in disused mines, and a budget—none of which has yet produced the substance. Rubin herself, who was nobody's fool, said in print that she would rather learn that gravity needed fixing than that the universe was filled with an invisible particle. Nobody consulted her.

Here is her life and her argument: the girl at the window in Washington, the graduate program that would not mail her a catalog, the December morning when the newspapers described a young mother with a theory of the universe, the image tube that made everything possible, and the fifty-year afterlife of a result she deliberately underclaimed.