What is dark matter?
An invisible substance thought to make up a quarter of all the “stuff” in the universe, dark matter leaves its gravitational fingerprints all over the cosmos. But despite decades of trying, scientists have failed to capture a single speck of dark matter, in part because they don't have a clear idea of what it actually is.
But what if the solution to the mystery of dark matter is that dark matter doesn't actually exist? What if this ghostly stuff is just a phantom of astronomers’ imaginations? Could there be another answer to the puzzles dark matter was invoked to solve?
Since the 1930s, astronomers have suspected that galaxies contain more mass that we can account for. That’s because, when astronomers clock the speed of stars circling around the center of the Milky Way and of galaxies moving in distant clusters, they all seem to be going too fast. They are going so fast that they should overtake the force of gravity tugging them inward and fly out into the void beyond. Yet something holds them back.
That “something,” most astronomers believe, is dark matter: matter we can’t see yet which has enough mass to keep those speeding stars in stable galactic orbits. But what is dark matter? Scientists have largely ruled out all known materials. The consensus is that dark matter must be a new species of particle, one that interacts only very weakly with all the known forces of the universe except gravity, with which it interacts as strongly as ordinary matter does. Dark matter is invisible and intangible, its presence detectable only via the gravitational pull it exerts.
But not every astronomer is satisfied with this interpretation. Some, like Stacy McGaugh at the University of Maryland, College Park, believe that the definition of dark matter is so slippery that it is impossible to prove or disprove. Researchers might be able rule out the existence of any specific conjectured form of dark matter particles, but "we cannot falsify the concept, so if one fails, we are free to make up another," says McGaugh. "This cycle can be endless — as long as we're convinced as a community that it has to be dark matter, we won't take alternatives seriously, but we can never be disabused of the concept of dark matter."
Instead of relying on mystery particles, a small community of researchers suggests an intriguing alternative: What if the answer lies in changing what we know about the laws of gravity? The leading alternative to dark matter is known as Modified Newtonian Dynamics (MOND). The assumption is that at large scales, the laws of gravity are different from Einstein's theory of general relativity. "MOND merely tweaks the way a known force, gravity, works—we don't have to accept that the universe is filled with invisible mass," McGaugh said.
In general, by tweaking Newton's laws of gravity when it comes to orbits at large scales, MOND predicts the velocities of stars within galaxies even better than dark matter does. "It works so well it seems there must be something to it," McGaugh said. MOND works especially well on a class of galaxies known as low surface brightness galaxies, very faint galaxies without bright centers, explains theoretical cosmologist Priyamvada Natarajan at Yale University. "It's better than dark matter at explaining the rotation curves of these galaxies, the speeds at which stars in a galaxy orbit the center."
However, critics point out that dark matter beats out MOND on other astronomical puzzles. "The biggest problem is perhaps clusters of galaxies—though MOND works well in individual galaxies, it doesn't fit clusters terribly well," McGaugh said.
In fact, even with MOND, there is still a need for dark matter. "The need for dark matter in such a theory is horrible," McGaugh said. "On the other hand, it is a fairly limited problem in scope—we believe there is more than enough ordinary matter in the universe that is yet undetected that would easily suffice to make up the difference."
Skeptics of MOND, however, point at the Bullet Cluster, two colliding clusters of galaxies. There is a clear separation of luminous and unseen matter seen there exactly matches what one would expect with the dark matter model—dark matter, being largely intangible even to itself, would "feel" the forces of the collision very differently than ordinary matter. MOND advocates say that although unseen matter could be involved, it might again be unseen forms of ordinary matter.
Maps of the cosmic microwave background—radiation left over from the Big Bang—also provide strong support for dark matter. Temperature aberrations seen in the cosmic microwave background seem to reflect the presence of both ordinary matter, which interacts with both matter and radiation, and dark matter, which influences matter but is essentially invisible to radiation.
So MOND advocates have a difficult task: Their theory must explain all the puzzles that dark matter has already solved, and it must present a new way of accounting for everything Einstein's theory of general relativity currently explains. For instance, general relativity proposes that matter and energy curve spacetime, creating the effect we know of as gravity. Massive bodies curve spacetime enough to visibly bend light, an effect known as gravitational lensing that astronomers have witnessed for decades. "We cannot explain the phenomenon of gravitational lensing without general relativity, and this is where MOND spectacularly fails," Natarajan said.
"It has proven hard to construct a relativistic version of MOND,” acknowledges McGaugh. “If one is going to introduce a new theory, it has to encompass existing, successful theories."
Meanwhile, physicists continue the quest to directly detect dark matter particles. "There are no significant results yet, but I am optimistic," says Natarajan. "In any case, I'm quite comfortable as it is with the evidence for the existence of dark matter."
But until physicists actually “see” a dark matter particle, researchers will continue to investigate alternatives to the dark matter model. "It could be wrong," McGaugh says. "We do not understand all there is to understand yet—there do remain fundamental mysteries to explore."
This is the first part of a two-part series on critics of dark matter and dark energy. Return next week for a look at alternatives to dark energy.
Editor's picks for further reading
FQXi: Out of the Darkness
Physicist Glenn Starkman is evaluating alternatives to general relativity.
NOVA scienceNOW: The Dark Matter Mystery
In this video, explore the evidence for dark matter.
Scientific American: What if There is no Dark Matter?
Could modifications to the theory of gravity eliminate the need for dark matter?