After Decades of Searching, Are Physicists Closing In on Dark Matter?

The universe is made up of much more than meets the eye. While telescopes reveal countless galaxies, each containing billions of stars, physicists and astronomers believe that visible matter is just the tip of the iceberg, so to speak, and that some kind of unseen dark matter must be out there as well, accounting for some 85 percent of the mass of the universe. No one knows what dark matter is made of, but scientists are confident it’s something that doesn’t interact with electromagnetic radiation, such as light—or else we’d be able to see it. But decades of searching have failed to yield any direct detections of this dark matter, leaving researchers wondering if they need to broaden their search strategies, or perhaps even rethink how gravity works.

The case for the existence of dark matter goes back to the 1930s, when astronomers analyzed the rates at which galaxies rotate and found there isn’t enough visible matter to account for the observed spin-rates. These so-called rotation curves, which plot the speed at which stars are moving as a function of their distance from a galaxy’s center, couldn’t be accounted for based on the amount of stars, gas and dust that’s visible within each galaxy.

Since then, further evidence has come from examining clusters of galaxies. The shape of a cluster can be distorted because of the gravitational influence of unseen matter between Earth and the cluster. This effect, known as “gravitational lensing,” lends further support to the notion of dark matter. Occasionally, galaxy clusters are observed slamming into each other; careful observations of the dynamics of the collision can reveal the presence of unseen matter accompanying both members of the pair. This effect can be seen most dramatically in the so-called Bullet Cluster, a pair of colliding galaxy clusters located some 3.7 billion light-years from our Milky Way, which appears to show the result of a cluster-on-cluster collision. Computer simulations of the collision suggest dark matter drove the process just as much as regular matter. Yet another line of evidence comes from observations of the cosmic microwave background, the radiation left over from the very early universe, which can be studied with radio telescopes. This radiation, which spans the entire sky, shows “hot” and “cold” spots—areas of more intense and less intense radiation—which are difficult to explain without invoking the idea of dark matter.

As compelling as these observations have been, they are all indirect; researchers would still like to snag dark matter particles directly.

For the last few decades, the leading theory has been that dark matter is made up of “weakly interacting massive particles,” or WIMPs—elementary particles thought to have been created some 14 billion years ago at the time of the Big Bang. Today these particles would be scattered about the universe, but because they interact only weakly with ordinary matter, they’re incredibly hard to detect. And while many sophisticated experiments have been searching for WIMPs, no definitive trace has turned up—leading some scientists to wonder if dark matter may be made of something else altogether.

“I think WIMPs are falling out of favor,” says Sean Tulin, a theoretical physicist at York University in Toronto. While the search for these elusive particles continues, he says many of his colleagues “are very happy to explore other alternatives.”

Scientists are now turning to a wider array of search strategies—and a longer list of potential dark matter candidates—in an effort to crack the almost century-old mystery.

While no laboratory dark matter searches have yet succeeded, physicists have managed to restrict the range of masses that a particle of dark matter might have. In August, researchers at the LUX-ZEPLIN experiment, located at the Sanford Underground Research Facility in South Dakota, announced that they had ruled out WIMPs with masses greater than about ten times that of a proton. The results are about five times more sensitive than any previous WIMP search.

The LUX-ZEPLIN result “is a beautiful, technical tour de force,” says Tracy Slatyer, a theoretical physicist at the Massachusetts Institute of Technology. “It’s remarkable that they were able to push the limit down this far.”

While the LUX-ZEPLIN result rules out heavy WIMPs, it is still possible that lighter WIMPs could be out there. And while more sensitive experiments will continue to search for lightweight WIMPs, they will inevitably come up against a natural limit: Eventually, such experiments would be so sensitive that they’d detect neutrinos, nearly massless subatomic particles created in the core of the sun and in other high-energy astrophysical environments. Researchers refer to this limit as the “neutrino floor.”

“Eventually we’ll be at the point where the background [signal] from neutrinos actually swamps the signal from the dark matter,” says Miriam Diamond, an experimental physicist at the University of Toronto. When physicists get to that stage, any dark matter particles that might be detected will be lost in a sea of neutrino detections.

As researchers get ever closer to this neutrino floor, they’re naturally thinking about other dark matter candidates besides WIMPs.

“There was a point where people were pretty sure it [the dark matter particle] was the WIMP, and that was a nice story,” says Slatyer. “But I don’t think anyone believes anymore that it absolutely has to be the WIMP.”

Many other potential dark matter candidates have been put forward, from exotic particles known as axions to primordial black holes, to a hypothetical new kind of neutrino known as a sterile neutrino. Dark matter may also be made up of more than one kind of particle, with theorists suggesting the existence of an entire “dark sector,” consisting of multiple kinds of dark matter particles.

“We need to take into account the idea that what we call ‘dark matter’ might actually be multiple types of dark matter particles,” says Diamond. “It’s kind of like Pokémon. You have to catch them all.”

Among the non-WIMP candidates, axions may be the new favorite. Axions were first hypothesized in the 1970s, when physicists were developing the Standard Model of particle physics—the framework that describes the known fundamental particles and their interactions. The axion—if it exists—would solve certain puzzles involving the strong nuclear force, which binds atomic nuclei together.

Like WIMPs, axions are thought to have been produced in the very early universe. Over time they would clump together, with the increasing gravitational pull of these clumps guiding the evolution of galaxies—as dark matter is believed to do. But axions are thought to be even lighter than WIMPs, and thus are just as elusive and difficult to detect.

“Axions are naturally produced in the early universe with enough abundance to account for all of the dark matter today,” says Peter Graham, a theoretical physicist at Stanford University. “The fact that they are much lighter than WIMPs just means that their number density would have to be much higher, in order to have the observed energy density of dark matter.”

Today, various labs are searching for axions, but with no definitive results so far.

While physicists are hunting for dark matter in the laboratory, astronomers have their own strategies for looking for evidence of dark matter in deep space. Their observations suggest that most galaxies, including our own Milky Way, are surrounded by dark matter “halos”—spherical shells of dark matter that extend out far beyond the visible part of the galaxy. While these halos are invisible, galactic dark matter can still be studied indirectly. A new generation of space telescopes, for example, will search for signs of dark matter particles colliding with each other; such collisions would produce bursts of high-energy radiation that could be observed with gamma ray telescopes. Another strategy is to study ribbon-like swaths of stars known as “stellar streams” in the vicinity of our own galaxy. Tracking the positions and motion of these streams can reveal how dark matter is distributed in the galaxy.

“A lot of particle physicists are becoming astrophysicists, because that’s where a lot of interesting puzzles are, related to dark matter, and there’s also a lot of new data that’s going to be coming in,” says Tulin. “So there’s a huge amount of excitement in the astrophysics community.”

---

Another possibility—seen as somewhat of a long shot—is that dark matter doesn’t actually exist, and instead there’s something about gravity that we don’t quite understand. Our best theory of gravity is general relativity, developed by Albert Einstein just over 100 years ago; so far it has passed every test with flying colors. But that hasn’t stopped some theorists from suggesting that it ought to be tweaked: Perhaps if Einstein’s equations were slightly adjusted, the dark matter problem would simply go away. No WIMPs, no axions—just a slight tweak to some century-old equations. But physicists who have studied the evidence for dark matter say it’s not so simple. While a modified gravity theory might explain galactic rotation curves, they say there’s no straightforward way to account for the data from galaxy cluster observations, from gravitational lensing and from the cosmic microwave background, all of which point toward unseen dark matter.

“I think modifying gravity is appealing because it seems simpler than postulating a whole other sector of particles—but I think that’s really a false argument,” says Tulin. “The hoops that you need to jump through in order to have modified gravity work actually turn out to be a lot more complicated compared to just postulating that the universe has this extra component, which works really well to explain many different observations.”

For now, physicists appear to be both excited by the precision of the latest experiments—WIMPs, the longtime favorite, have still not been ruled out—and also frustrated by the lack of any conclusive laboratory results, even after decades of searching.

For many astronomers and physicists, making sense of dark matter is the most urgent problem driving their research. At the very least, solving the dark matter mystery would shed light on the fundamental physics of the universe, says Slatyer. “I think it would be a great accomplishment of human curiosity, if we were able to figure this out,” she says. “Obviously I would much prefer it takes seven years than 70 years.”

Dan Falk | | READ MORE

Dan Falk is a science journalist based in Toronto. His books include The Science of Shakespeare and In Search of Time.