Somewhat lost amid all of the publicity over the deaths of Carrie Fisher and Debbie Reynolds was the death, on Christmas day, of Vera Rubin. Vera was one of the most influential cosmologists of her generation, and a legitimate contender for the Nobel Prize. I only met her once, at a conference at Irvine in the early 1990s. (Tom Hanks was there as well -- evidently he was thinking of making a movie about cosmologists but decided we were too boring. A wise decision).
What did Vera discover? Her most important work has to do with the "rotation curves" of galaxies. If you observe the way that planets orbit the sun, you notice that their orbital speeds decrease as you look at more and more distant planets. (Mercury moves faster than the Earth, which moves faster than Mars, and so on). In a galaxy, things are more complicated. There isn't one big central mass like the Sun -- instead, you have lots stars doing a complicated dance around each other. But (and here's the key point) far enough away from the center of a galaxy, any object should feel the force of the galaxy as a whole -- it should orbit the whole galaxy in just the same way that the planets orbit the sun. And that means that stars and gas orbiting far away from the galaxy should move more slowly as you look farther and farther out.
But that's not what Vera Rubin observed at all. Instead, she noticed that objects orbiting a galaxy all tend to move with the same speed as they get farther and farther from the center. If you graph the speed versus distance, you get a horizontal line -- hence the expression "flat rotation curves." And the best explanation for this is that there is extra matter in the galaxy, in the form of dark matter, that pulls on orbiting objects, making them move faster than expected.
Vera never won the Nobel Prize -- the Prize committee is notoriously conservative, and they prefer ironclad proof of new discoveries. And there's always the possibility that Rubin discovered something other than dark matter. Maybe instead of dark matter filling the galaxy, the theory of gravity itself breaks down on large scales? I wouldn't bet on it, but you never know....