Is Dark Matter Real? Inside the Theories That Leave This Mysterious Phenomenon Out

The universe is hiding something.

Stars at the outer edges of galaxies whirl around the galactic center far more swiftly than the laws of physics say they should. At even larger scales, galaxy clusters clump together in ways that should only be possible if the galaxies were more massive than they appear. And most of our models of how the Big Bang happened suggest that much more matter should have been created than we see.

Either there’s more to the universe than meets the eye, or the rules of physics work very differently than we think they do. Most astrophysicists and cosmologists (scientists who study the origins and evolution of the universe) today lean toward the first option: dark matter. But a few have devoted their efforts to finding a set of rules that could produce the universe we see, without dark matter.

Inverse spoke with experts in both fields about dark matter, the laws that make our universe work, and the quest to understand everything.

Hidden Mass or New Rules of Physics?

The things we can see in the universe — planets, stars, vast clouds of gas, and galaxies — make up only about 5 percent of what’s out there, according to physicists. Another 70 percent is dark energy, a little-understood force that’s driving the expansion of our universe (we know it’s expanding because astronomers have measured the way light waves from distant stars get stretched out as their sources accelerate away from us), and the remaining 25 percent is stuff called dark matter.

Dark matter doesn’t absorb, emit, or reflect light, and it doesn’t seem to interact with normal matter in most ways; theoretically, you could walk through a wall made of dark matter and never see it or feel it. The only rule of physics that dark matter seems to follow is gravity: It has mass, apparently, and that mass has gravity and forms a vital part of the scaffolding of galaxies, galaxy clusters, and the universe itself at a large scale.

As explanations go, dark matter sounds weird, but it works well. The amount of “extra” matter created in the Big Bang seems to match the amount of unseen mass in the universe. It balances physicists’ equations nicely and explains what we see in the universe around us.

But because dark matter doesn’t interact with light or matter, scientists haven’t yet actually detected it directly; they can only see how its gravity affects the universe around it. That leaves a little sliver of doubt and inspires some physicists and cosmologists to look for other ways to explain those effects.

One of the most popular alternatives to dark matter is called Modified Newtonian Dynamics, or MOND, and it proposes that gravity works a little differently than Isaac Newton first described it. The farther away an object is, the weaker its gravitational pull feels. According to MOND, gravity’s effect weakens slightly less over distance than it does in Newton’s original equations. That, allegedly, explains why galaxies seem to spin fast, as if they have a lot more mass on their outer edges than it appears.

MOND, and other modified ways of describing gravity, “are quite good at describing the properties of galaxies, but usually fail at describing the large-scale structure of the Universe,” astrophysicist Sébastien Comerón tells Inverse. And to replace dark matter, any new model of how the universe works has to explain everything we see, at large scales and small ones.

A recent study, led by cosmologist Rajendra Gupta, tackled that problem. Gupta suggested earlier this year that the universe might be 26.7 billion years old — nearly twice as old as all our evidence so far suggests — and that the laws of physics are much less consistent than we thought.

“I was trying to understand the so-called 'impossible early galaxy' problem,” Gupta tells Inverse. When astronomers peer into the distant, early universe with the James Webb Space Telescope (JWST), they see galaxies that look more massive and more neatly structured than they should; just 1 billion years after the Big Bang, galaxies shouldn’t have had time to pull so much mass together. (The same is true of some of the earliest supermassive black holes in the universe.)

Those precocious early galaxies challenge what we think we know about how supermassive black holes and galaxies form and evolve.

But Gupta tackled the “impossible early galaxies” problem by developing a new model of the universe, which would explain the precocious early galaxies. His model hinges on a century-old theory called tired light, which suggests that light actually loses energy as it travels across space. It also leans on another old theory that suggests the laws that govern how our universe works (like gravity) aren’t so constant after all — they weaken over distance. According to Gupta’s model, those changes add up to differences in how light appears by the time it reaches JWST, making the universe look younger than it is.

The model suggested another conclusion as well: Dark matter and dark energy don’t exist.

“The model yields matter content just enough for the Universe's ordinary matter; there is no room for dark matter. In fact, there is no dark energy in this model either,” says Gupta. “Accelerated expansion of the Universe, attributed to the dark energy, is due to the weakening of the forces of nature in the new model.”

But there are some problems with the idea, starting with the fact that both “tired light” and the idea of varying physical constants (like gravity) fell out of fashion among scientists a long time ago because they didn’t fit with observations about how the universe behaved.

“I think it is an interesting idea to explain the origin of recently discovered compact bright galaxies at high redshift with just varying two new parameters that determine the property of photons. However, it remains to be seen whether the model can explain the other phenomena,” cosmologist Kaiki Inoue tells Inverse. Those other phenomena include some inconsistences in how the cosmic microwave background looks in different directions, as well as how galaxy clusters and even larger-scale structures form. “So, in my opinion, it is an interesting idea but not a fully developed model,” says Inoue.

Don’t Bet Against Dark Matter

“It is worth doing exploratory work outside the box,” says Comerón. “But the likelihood of this particular avenue being the one that solves the problems in cosmology is rather low in my opinion.”

It’s not clear yet whether modified gravity theories, like MOND, actually fit well with Gupta’s proposed model of the universe, either.

One of the best arguments in favor of dark matter may be its inconsistency. Comerón and his colleagues recently discovered a galaxy that apparently contains no dark matter — which is weird but not unheard of. And if we’re seeing not invisible, undetectable matter, but different laws of physics, those laws should apply to all galaxies in the same way.

“I have sometimes been attracted by the idea of alternative gravities myself, but the discovery of the odd properties of NGC 1277 leaves me little doubt that dark matter exists,” says Comerón. “If dark matter could be explained by a modification of gravities, it would be odd to have gravity modified in all galaxies except in a few. On the other hand, one could conceive mechanisms to remove dark matter from galaxies that have got a peculiar formation or interaction history.”

And that seems to be the consensus among most astrophysicists and cosmologists. So far, no one has figured out how to directly measure dark matter — and until they do, there will always be at least a little room for debate about its existence. But Inoue and his colleagues recently used gravitational lensing to map how dark matter is distributed along one narrow swath of the universe, and others are busily trying to work out exactly what it’s made of and how it behaves.

“I would not bet any money against dark matter,” says Comerón.