Teaming up with a young Carnegie colleague, W. Kent Ford Jr., Dr. Rubin set out in the early 1970s to map the distribution of mass in spiral galaxies by measuring how fast they rotated. The faster the stars were going around, the more gravity, and thus mass, must be keeping them in their orbits.

They expected to find that most of the mass was where most of the starlight was, at the centers of the galaxies. In that case, stars on the outer fringes of a galaxy should have been moving more slowly than those in the inner regions — the way Pluto, on the outskirts of the solar system, takes 248 years to go around the sun, while Mercury speeds around in 88 days.

To their shock, however, they found that the stars on the outskirts of galaxies were not slowing down; if anything, they were speeding up. By the laws of either Newton or Einstein, it meant that there was extra mass out there where there was relatively little light, mass that was speeding up the stars.

“Great astronomers told us it didn’t mean anything,” Dr. Rubin said. Told to look at more galaxies, they did, and the effect persisted.

In fact, the idea that there was more to the universe than could be seen had been lurking on the edges of scientific respectability since the 1930s, when the Caltech astronomer Fritz Zwicky deduced that some invisible “missing mass” was required to supply the gravitational glue that held clusters of galaxies together. Otherwise, with the galaxies moving so fast, a cluster would simply fly apart.

“Nobody ever told us all matter radiated” light, Dr. Rubin said. “We just assumed it did.”

Another boost to this idea had come in 1973, when the Princeton theorists Jeremiah Ostriker and James Peebles suggested, based on computer simulations, that spiral galaxies would warp and fall apart — because of gravitational forces from stars — unless they were embedded in a halo of dark matter, like a hamburger patty surrounded by a bun.

Dr. Rubin and Dr. Ford’s work brought these ideas to center stage.

“Vera’s work, mostly in the early ’80s, clinched the case for dark matter for most astronomers,” Dr. Ostriker wrote in an email, noting that she had been working with familiar galaxies and the kinds of optical observations that astronomers understood.