Our understanding of gravity can explain why an apple falls from a tree and how our planet orbits the sun. But when it comes to the most gigantic things in the universe — from galaxies on up — our current theories fail us. The leading contender for explaining why big things don't behave like they should is a theoretical substance called dark matter. But we've never directly seen it, and worse, it doesn't fully explain the mystery. There's a vocal minority that says dark matter doesn't exist at all. Instead, it's our understanding of gravity that needs tweaking. Who's right?
One of These Things Is Not Like the Other
The idea that there's something invisible messing with the universe's largest structures didn't come out of nowhere. Like many things in science, the conception of dark matter was the result of many incremental steps. One of the most famous early examples of a moment when scientists spotted an object that didn't act like it should was when Uranus was discovered in 1781. Isaac Newton's laws made firm predictions for how a planet this far from the sun should move, and yet it refused to follow the rules, moving too fast for several decades, matching the predictions for a few more, then moving too slowly after that.
Some thought this had to be because the law of gravitation was fundamentally flawed; others thought there might be another body in the solar system throwing a wrench into things (just as the odd orbits of several Kuiper belt objects are making modern astronomers think there might be a ninth planet today). In the end, astronomers did find another body — Neptune — and Newton's laws remained safe.
But later, when astronomers noticed that Mercury's orbit was also a bit off, there was no new body to blame. It took a new theory instead: Einstein's general theory of relativity, which explained that massive objects like the sun warp spacetime. This accounted for that quirk in the orbit of Mercury, since it's so close to the sun's gravitational warp.
But in the grand scheme of the universe, planetary orbits are small potatoes. In the 1930s, scientists were much more flummoxed about galaxies behaving badly. You see, most of the stars in a spiral galaxy are concentrated near the center, so it was reasonable to assume that that's where most of the mass, and thus the gravity (thanks, Einstein!), was concentrated, too. Just as Pluto orbits the sun more slowly than Mercury does, the further from the center of the galaxy a star is, the slower it should orbit.
But that wasn't the case. The stars farthest from the centers of their galaxies moved just as fast as those closer in. In the 1960s, astronomers Vera Rubin and Kent Ford concluded that it was because of some unseen mass, or "dark matter," making this happen — about 10 times as much as the ordinary matter we can see.
We still haven't seen this "unseen" mass — and because it doesn't absorb, reflect, or emit light, we never will — nor have we directly detected it in any other way, but we've seen countless pieces of evidence for its existence. Its mass warps spacetime to turn galaxies into magnifying glasses, and its effects leave traces in the light left over from the birth of the universe known as the cosmic microwave background. Today, scientists estimate that dark matter makes up a full 27 percent of the matter in the universe. Ordinary matter makes up less than five percent.
In 1983, an Israeli physicist named Mordehai Milgrom proposed an alternative solution: Maybe there's no invisible matter. Maybe Newton was just wrong — or at least wrong-ish. His theory of modified Newtonian dynamics (MOND) suggested that we tweak Newton's second law of motion, which says that an object accelerates in proportion to the force placed upon it. Milgrom said that maybe these laws change in certain circumstances, such as when a star is far from its galactic center. If that were true, you could explain their speed without invoking the existence of invisible matter.
Since then, numerous papers have shown that modifications to gravity can actually yield the exact behavior we see in galaxies. Individual galaxies, that is. Once you take in the bigger picture, things go a little haywire. MOND models show that colliding galaxies and galaxies within clusters don't behave the way they should, and the patterns in the cosmic microwave background don't match up either. It's a bit like pulling yarn to fix a hole in a sweater and destroying the rest of the sweater in the process.
Still, the dark matter camp has its challenges, too, mostly because we can only simulate so much of the universe. "Dark matter simulations often contain trillions of particles now, and try and take into account photon pressure, star formation, supernovae and other feedback effects," Ethan Segal writes in Forbes. "But each individual galaxy is estimated to contain somewhere between 10^60 and 10^80 dark matter particles; a trillion is only 10^12." For small and average-sized galaxies, that leaves a factor of more than a million particles left unaccounted for, so we can't prove their behavior is due to dark matter.
"The great challenge for modified gravity is to reproduce the successes on large-scales of modern cosmology; the challenge for dark matter is to reproduce the details of the smallest scales correctly," writes Segal. But in terms of evidence, dark matter is winning — even if you have to take it with some healthy uncertainty.
This article first appeared on Curiosity.com.