In the 1970’s Vera Rubin didn’t set out to upend modern cosmology. She was just always curious about the heavens. It started with building a homemade telescope out of cardboard and glass, and it progressed with her becoming the only astronomy undergraduate student at Vasser College, graduating in 1948. She was qualified enough to get into Princeton, except for the fact that she was a woman, and so they wouldn’t let her in. Despite years of discouragement and harassment, she made a name for herself in cosmology, joining the first generation of scientists to piece together the large-scale structure of the universe.
But it was her observations of the Andromeda galaxy that changed everything. She became interested in studying the motions of stars and gas clouds within that galaxy. Nobody expected something radical to come out of the measurements, but it was worth doing anyway. It’s a classic case of science by sheer exploration: go out and measure something and see if something interesting comes up.
And something interesting came up.
She was especially interested in the rotation rates within the galaxy as a function of distance from the center. The rotation rate at a given distance should neatly line up with the amount of matter contained within that distance, and how great that distance is. This is Kepler’s laws (and later Newton’s laws and then Einstein’s general relativity) at work: the orbital speed is related to the amount of matter within the orbit, just like the Earth’s orbital speed is dictated by the mass of the Sun and our distance from the Sun.
It's relatively easy to measure the amount of matter within a given orbit, especially with something like the Andromeda galaxy that shows a good fraction of its face to us. We just measure the brightness, which is proportional to the number of stars, gas clouds, and other generally glowy things inhabiting a galaxy. And by just looking at a picture of galaxy you can tell that most of the stuff is concentrated in the center, and then just sort of tapers off from there.
So we expect that orbital speeds should be really, really high near the center, and then just sort of taper off from there. At the very edge of a galaxy, you are very far away from all the stuff, so you should have a slow and lazy orbital speed.
Except, as Rubin discovered, you don’t. The very edges of the Andromeda galaxy were spinning almost as quickly as the middle parts, and even the center. In fact, the rotation rate across the galaxy was almost constant. And it wasn’t just some galactic fluke of Andromeda; Rubin went on to measure dozens of galaxies and consistently got the same result.
It appeared that the material in every galaxy was rotating far too quickly given the amount of matter that we can see. This is the essential observational reality of the whole dark matter idea, a reality that crops up again and again. At large scales, nothing ever seems to line up. There’s an extra source of gravity, some extra glue, that’s holding galaxies together.
By all rights Rubin should’ve won the Nobel Prize. And by now just about everybody agrees that if we were talking about a Victor Rubin, the prize would’ve been his. But hey, in 2025 Vera will be featured on a special edition of the US quarter, which I guess is going to have to be the consolation prize.
Anyway, it’s not just galaxies. All the way back in the 1930’s Swiss-American astronomer Fritz Zwicky – who really knew how to rock a bolo tie, and for that I applaud him – was studying the Coma cluster, a nearby cluster containing over 1,000 individual galaxies. In Zwicky’s case, he found that the galaxies themselves were buzzing around far too quickly for the mass that he could observe. The cluster should have simply dissolved eons ago, with the individual galaxies just flying away from each other. But there the Coma cluster was, just serenely sitting there like there wasn’t a problem at all.
For his work Zwicky earned himself naming rights. He suspected, but could not prove, that the Coma cluster was bound together by a hidden source of matter, a “dunkle materie” in German – dark matter in English. Despite the mystery, he and the rest of the astronomical community would put the problem down for later generations to solve.
By now, we are further removed from Rubin’s discovery than she was from Zwicky’s. Which means we’ve had a lot of time to think about what dark matter could be, and a lot of time to go looking for it in the universe. And to be frank, we see it everywhere.
Well, I mean, we can’t see it, because it’s invisible, but we see evidence for it every time we make any sort of observation of the large-scale universe. The evidence that something funky is going on with the universe is…substantial, to say the least. I could devote an entire SERIES of articles on the evidence for dark matter (don’t tempt me!), but let me give you a quick rundown.
By now, we’ve measured tens, maybe even hundreds, of thousands, of rotation curves of every kind of galaxy. They all rotate with speeds far too great to explain with the matter what we can observe. We’ve measured thousands of galaxy clusters with the same result.
There’s even funkiness in the cosmic microwave background. This is the radiation leftover from when the universe became a neutral gas when it was only 380,000 years old. This radiation completely covers the sky (and is responsible for over 99.9999% of all the radiation currently in the universe). It’s almost perfectly uniform, with the exact same brightness and temperature, but it does have tiny one-part-in-a-million differences, known as anisotropies. Those differences tell us what the universe was up to all those billions of years ago. We can create models of that early universe, plugging in various kinds of ingredients and forces and physics and whatnot, and compare that to what we see.
No matter how you massage the math, no matter what exotic chicanery you concoct, you can’t replicate the signal of the cosmic microwave background without including some additional source of hidden matter. You just can’t.
