The search for dark matter has been blown wide open
AI News Desk
·
MIT Technology Review
··
14 min read
Underneath an Apennine massif, below the Jinping Mountains of Sichuan, and at the bottom of a South Dakota mine, there is a cosmic hunt afoot.
Underneath an Apennine massif, below the Jinping Mountains of Sichuan, and at the bottom of a South Dakota mine, there is a cosmic hunt afoot.
Isolated deep beneath these rocky shields, massive detectors filled with liquid xenon aim to make the first direct detections of dark matter, the long-sought invisible substance whose gravity has sculpted our universe.
The hope is that someday, a bit of dark matter called a weakly interacting massive particle (a WIMP, for short) will collide with a xenon atom, creating a burst of light and electric charge. After running for years, these experiments have recently begun seeing infrequent blips from a particle that glides ethereally through ordinary matter until it crashes into the detectors. Unfortunately, the new signal is not produced by dark matter. Instead, the detectors are picking up on something similarly insubstantial but much more mundane: neutrinos, the featherweight subatomic particles that the sun and other stars produce in massive quantities.
Physicists’ failure to find dark matter where they thought it was has led to a cornucopia of proposals for new ways to search: quantum sensors, liquid-helium-based detectors, searches in Jupiter’s atmosphere, and more.
Physicists have known for decades that this neutrino background was there; they were just hoping to discover WIMP dark matter first. Now the chance is looking slim. Some of today’s WIMP detectors are simply so large and sensitive that they are entering the so-called “neutrino fog,” in which the ordinary particles are likely to drown out any signal from the main target. There is no shielding these detectors from neutrinos, which easily slip through the Earth itself. That means the next experiment to use this long-standing approach for seeking WIMP dark matter may be the last.
Hitting the neutrino fog does not, however, mean an end to the search for dark matter. Researchers just have to shift the focus of their hunt. “We haven’t seen WIMP dark matter,” says Kathryn Zurek, a theoretical particle physicist at the California Institute of Technology. Nor, she says, have scientists found new particles in the Large Hadron Collider (LHC), the powerful proton-smashing facility that straddles the border between France and Switzerland. “And so people naturally broaden their scope,” Zurek says. As they do, there are plenty more candidates waiting in the wings
In other words, the hunt is transforming from a narrow probe into a kind of free-for-all. It’s a big shift. Today, particle physicists are less sure about dark matter’s identity than when they began looking for it. They’ll freely admit that they cannot presume the basics—for example, if the stuff that makes up dark matter is heavier than the Earth or lighter than a radio wave, or if dark matter is one kind of particle or a dozen.
The uncertainty can be frustrating, even humbling. “The potential range where the candidates could be is so enormous that the odds of any one small experiment finding it are very, very small,” says Hugh Lippincott, a dark matter experimentalist at the University of California, Santa Barbara.
But physicists’ failure to find dark matter where they thought it was has also led to a cornucopia of proposals for new ways to search: quantum sensors, liquid-helium-based detectors, searches in Jupiter’s atmosphere, and more. “Now there’s a great deal of excitement. And finally, there’s technology there,” says Gray Rybka, a University of Washington physicist who co-leads an experiment looking for axions, an ultra-lightweight dark matter candidate.
Still, with so many places to look, where does it make sense for physicists to begin again?
For starters: the birth of the universe. Dark matter has been with us since the beginning, and there’s much to learn from those early eons. Maps of the cosmic microwave background—the first light from the universe’s early years—are full of fluctuations caused by the clumpiness of underlying matter. Reading these cosmic dregs, researchers can tell that only 17% of the matter in the universe is made of ordinary particles like protons and neutrons. The remaining 83% is dark matter, which has little to no interaction with light or ordinary matter other than through gravity.
We can tell quite a bit about dark matter from those gravitational effects. We know that the Milky Way contains a halo of the stuff. Our own solar system orbits the galactic center far too quickly to be bound by the tug of ordinary matter alone: without dark matter’s gravitational tether, we would be flung off into intergalactic space. We can also see how the heft of a galaxy’s dark matter bends the path of light as it makes its way to Earth’s telescopes. And on the grandest scale, we can see how superclusters of galaxies are distributed in space like dewdrops on a spiderweb. No cosmological theory without dark matter can explain all these phenomena.
But all the astronomical and cosmological evidence has little to say about what dark matter is actually made of. “It does not tell you anything about the individual constituents. It just tells you the effect of a bunch of them together,” says Lippincott, who has led the LZ experiment, a WIMP dark matter detector currently in operation at the former Homestake Mine in South Dakota.
The idea of WIMPs emerged during the 1980s. At the time, theorists were exploring add-ons to the standard model, the overarching theory of particle physics that describes all the universe’s fundamental particles and their interactions. The standard model is powerful but doesn’t account for everything—notably, it omits gravity—so some adjustments seemed necessary. The most popular idea, a class of theories called supersymmetry (SUSY, informally), called for pairing each known particle type in the universe with an as-yet-unseen “superpartner.” To have avoided detection, superpartners would have to have a lot of mass (putting them outside the reach of existing colliders) and be weakly interacting, able to pass ghostlike through matter. That is to say, they would be WIMPs. It didn’t take too long for physicists to realize that the WIMP was also an excellent dark matter candidate: two problems, one particle.
The appeal of SUSY was so strong that many particle physicists expected to see WIMPs as soon as the LHC turned on in 2008. Instead, as the data came in from the LHC, the most promising SUSY theories were largely ruled out.
WIMPs, though, have lived on, no longer tied to the theory that birthed them. And the latest generation of dark matter detectors have kept the hunt alive. After all, Lippincott says, “the motivation to look for dark matter has not gotten any weaker, right?”
Now it’s looking as if those WIMPs—if they exist—may be beyond our current powers of detection. There are a range of difficulties, but the most pervasive is that when you’re looking for a needle in a haystack, even a few other needle-shaped objects can cloud the search. Interactions between neutrinos and the xenon inside the detectors, while astronomically rare, do just that.
A future, final WIMP experiment would investigate the rest of the places WIMPs could be hiding, even peering into the neutrino fog. An effort called XLZD (a somewhat ungainly acronym reflecting a mashup of existing collaborations) would use 60 to 80 metric tons of liquid xenon, which is about the yearly global production of that rare element and at least six times more xenon than the biggest current detector contains. But it may already have been scuttled, for reasons unrelated to the neutrino fog: At a particle physics meeting in December 2025, the US Department of Energy announced that the US would neither host XLZD nor pay its share of the price tag, which could be well over $300 million. “It may be that the project doesn’t happen at all,” Lippincott says. “And then the US pulling out would have effectively killed it.”
In the meantime, the hunting ground for dark matter has been expanding dramatically. In 2022, researchers developed an enormous plot showing various candidates for what dark matter is made of and their possible masses. The options fell mainly into two ranges that span about 50 orders of magnitude (that’s 10 50 , or 10 with 49 more zeroes). At the heavy end of the scale are primordial black holes, hypothetical asteroid-size objects that formed shortly after the Big Bang and might still be floating about the universe.
But many physicists are most interested in the lightest option: the axion.
Like the WIMP, the axion first emerged as a solution to problems with the standard model. In the axion’s case, it was to address an outstanding question about the strong nuclear force, the fundamental force that holds atomic nuclei together. Axions were proposed by theorists in the 1970s as a mathematical tweak. By adding a particle with a trillionth to a millionth the mass of an electron, they could explain why the strong force seems to behave precisely the same way when it comes to both matter and antimatter even though it doesn’t need to—an unanswered question known as the strong CP problem.
Axions would be abundant and interact infrequently with ordinary matter, two necessary features for dark matter. Detecting axions is no walk in the cosmic park, though. The delicate particles carry only a hint of energy—about as much as a radio wave. That makes them imperceptible to traditional particle detectors. (Proton collisions at the LHC, for example, pack about a quintillion times more energy.)
Physicists have developed strategies they hope will bear fruit. One of the most promising ideas is to use an ultracold chamber suffused with a strong magnetic field like a radio, tuning it to specific wavelengths. If an axion happens to be resonant, meaning it has the same wavelength as the chamber, it stands the chance of transforming into something far easier to detect: a particle of light. The first full-size detector, called a haloscope, was built at Lawrence Livermore National Laboratory in 1994; today there is a coterie of detectors around the world, with quirky acronymic names like MADMAX and ABRACADABRA.
Fiddling with a cosmic radio to listen for a rare invisible particle is tricky and requires quantum sensors cooled to millikelvin temperatures, a fraction of a degree above absolute zero. Even that doesn’t entirely negate background noise. “At one point, we detected a ‘message from God’ in our experiment,” Rybka tells me wryly. “We looked it up in the FCC allocation of spectrum. It was a religious programming station.”
So far, particle physicists have combed somewhere between 10% and 20% of the parameter space—the area on a chart showing how massive and interactive dark matter might be—where an axion that solves the strong CP problem might exist. But the search for axions might not end there. Physicists are also looking for axions that don’t address the strong CP problem but could still be dark matter. “More and more people are thinking about models specifically for dark matter, even without connecting that to any other problem,” says Stefania Gori, a theorist at the University of California, Santa Cruz. “Of course,” she says, “if one can solve more than one problem at once, it’s even nicer.”
As dark matter has continued to go undetected, physicists have dropped some of their theoretical desiderata. Modern candidates don’t need the convenience of WIMPs or the clean simplicity of axions; they need only fulfill the requirements for dark matter.
The poster child for these unpretentious candidates is low-mass dark matter, so named because it is somewhere between an electron and a proton in weight. If a WIMP is a billiard ball, low-mass dark matter is a ping-pong ball, explains Rouven Essig, a theorist at Stony Brook University. Like a ping-pong ball hitting bowling pins, low-mass dark matter wouldn’t have enough heft to produce a clear signal from a collision with atomic nuclei. “One needed to really come up with new ideas for how one could detect the signals, and then, of course, also new technologies,” says Essig.
Researchers have designed novel detectors and installed them in underground labs around the world—often right next to their WIMP-detecting forebears. Some of the new machines look for evidence that particles have collided with electrons and ionized them. Others peer into crystals whose lattices should jiggle subtly after a bump from such a particle. There are even prototypes that look for hints in liquid helium, a sensitive superfluid that should throw up a splash when hit by incident dark matter.
There is, however, a rub: noise. While all experiments in dark matter detection are hampered by noise from the outside world, low-mass searches also suffer from the intrinsic din of their detector mediums. Atomic lattices like those inside crystals are like a crowded subway car, naturally prone to shaking and jostling their electron passengers. It’s not the quiet space you want if you’re looking for dark matter.
This noise has created challenges. In 2020, a surprising number of what physicists call “excess events” cropped up across a range of detectors looking for low-mass dark matter. Could they, some physicists wondered, be a signal of dark matter? Unfortunately, the noise causing most of these readings has been identified, and the answer is a pretty firm “no.”
Background noise can come from anywhere: impurities in silicon-based detectors, materials that have spent too long on Earth’s surface (cosmic rays make them lightly radioactive). In one experiment, a crystalline detector was clamped too hard; the extra pressure caused vibrations that looked like evidence of dark matter. “It’s always been true that understanding those backgrounds has been difficult,” says Dan McKinsey, an experimentalist at Lawrence Berkeley National Laboratory. “But we’ve shifted our regime so quickly that suddenly we don’t understand, as a community, what the key backgrounds are.”
Getting a handle on that background noise is key, especially as experiments to detect low-mass dark matter get bigger. In a few years, for example, McKinsey and his colleagues plan to install a number of tabletop experiments in Modane, France, underneath 1,700 meters of solid rock on the Italian border. One of them is a vessel with about a tablespoon of ultracold liquid helium. If a particle of low-mass dark matter impacts the liquid, it will generate a vibration that sprays thousands of helium atoms upward, where silicon detectors will look for microscopic changes in voltage. Other setups will work with sapphire-silica crystals and gallium arsenide, a semiconductor.
These experiments will help researchers identify the best approaches to use for bigger, more sensitive—and more expensive—detectors. “There’s still lots of ideas, and it’s still not quite clear what’s going to be the best one to really scale up,” Essig says. For now, if it fits on a table and can plausibly detect dark matter, physicists are willing to try it.
The hunting ground for dark matter extends past the surface of any table, or even Earth itself. Some researchers have suggested we look for the stuff not in underground laboratories but on planets, stars, and moons. If dark matter particles occasionally annihilate when they encounter one another, they could ionize hydrogen in the atmosphere of a planet, creating an ultraviolet aurora visible from space. This self-annihilation could also be a strong source of heat—enough to melt a planet’s core. The fact that Earth’s core is solid provides limits for dark matter’s characteristics, and measuring the temperatures of planetary cores more precisely could set even more stringent constraints.
Searching for astrophysical signs of dark matter is not new, but in recent years physicists have become almost artistic with their proposals. One particularly evocative suggestion: looking at the icy ocean of Jupiter’s moon Ganymede. If dark matter is indeed really heavy—possibly a primordial black hole—it could punch through the surface and leave a crater that looks very different from one caused by an asteroid.
Some particle physicists think it would be better to drop all the existing assumptions and refocus. “Everything we know about dark matter has come from its interaction with gravity,” says Caltech’s Zurek. If you concentrate more specifically on that interaction, she says, at least “you’re guaranteed to learn something.” We know how dark matter behaves on the scale of the observable universe as far down as galaxies. “Below that, we really know very little about how dark matter collapses under the weight of gravity,” Zurek says. How, for example, does it clump up on the level of a solar system?
In recent years physicists have become almost artistic with their proposals. One particularly evocative suggestion: looking at the icy ocean of Jupiter’s moon Ganymede.
This isn’t a “next year” or “next five years” project. The technologies physicists have now are simply not sensitive enough to search for these gravitational interactions. So Zurek is thinking long term. Really long term. “It’s going to take decades, like probably 100 years,” she acknowledges. “It may not be something that I see in my lifetime.”
Someday—perhaps by monitoring the timing of distant pulsars (the spinning remnants of dead stars) or measuring slight perturbations in gravitational interaction between atoms suspended in lasers—physicists could learn more about dark matter’s true nature.
For now, the scale of the problem seems daunting, especially compared with the ones particle physicists have tackled in the past. Before the LHC even turned on, for example, those hunting for the Higgs boson had hemmed their quarry in. They knew, from rigorous theoretical calculations and robust experimental measurements, that if the boson existed, it had to weigh between 120 times and 150 times the mass of a proton. Soon after the LHC began smashing protons, the Higgs popped out of the data, right at 133 times the mass of a proton.
Dark matter, by contrast, remains an almost total mystery. Rybka likens guessing about its mass or its interaction strength to “drawing numbers out of a hat.” “We literally don’t even know what the hat looks like,” he adds.
With this many unknowns, success is far from certain, and the researchers have no illusions. “There’s no discovery guaranteed. You might be wasting your time completely,” Essig says.
None of this has dissuaded him or others. “That’s just the nature of the problem. We have to search far and wide and explore lots of things,” Essig says. “If you don’t like it, do something else.”
Dan Garisto is a freelance physics journalist based in Syracuse, New York.
