Physicists\u2019 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\u2019s atmosphere, and more.<\/strong><\/p>\n<\/blockquote>\n 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\u2019s WIMP detectors are simply so large and sensitive that they are entering the so-called \u201cneutrino fog,\u201d 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.\u00a0<\/p>\n 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. \u201cWe haven\u2019t seen WIMP dark matter,\u201d 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. \u201cAnd so people naturally broaden their scope,\u201d Zurek says. As they do, there are plenty more candidates waiting in the wings<\/p>\n In other words, the hunt is transforming from a narrow probe into a kind of free-for-all. It\u2019s a big shift. Today, particle physicists are less sure about dark matter\u2019s identity than when they began looking for it. They\u2019ll freely admit that they cannot presume the basics\u2014for 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.\u00a0<\/p>\n The uncertainty can be frustrating, even humbling. \u201cThe potential range where the candidates could be is so enormous that the odds of any one small experiment finding it are very, very small,\u201d says Hugh Lippincott, a dark matter experimentalist at the University of California, Santa Barbara.\u00a0<\/p>\n But physicists\u2019 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\u2019s atmosphere, and more. \u201cNow there\u2019s a great deal of excitement. And finally, there\u2019s technology there,\u201d says Gray Rybka, a University of Washington physicist who co-leads an experiment looking for axions, an ultra-lightweight dark matter candidate.\u00a0<\/p>\n Still, with so many places to look, where does it make sense for physicists to begin again?\u00a0<\/p>\n For starters: the birth of the universe. Dark matter has been with us since the beginning, and there\u2019s much to learn from those early eons. Maps of the cosmic microwave background\u2014the first light from the universe\u2019s early years\u2014are full of fluctuations caused by the clumpiness of under\u00adlying 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.<\/p>\n 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\u2019s gravitational tether, we would be flung off into intergalactic space. We can also see how the heft of a galaxy\u2019s dark matter bends the path of light as it makes its way to Earth\u2019s 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.\u00a0<\/p>\n But all the astronomical and cosmological evidence has little to say about what dark matter is actually made of. \u201cIt does not tell you anything about the individual constituents. It just tells you the effect of a bunch of them together,\u201d says Lippincott, who has led the LZ experiment, a WIMP dark matter detector currently in operation at the former Homestake Mine in South Dakota.<\/p>\n 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\u2019s fundamental particles and their interactions. The standard model is powerful but doesn\u2019t account for everything\u2014notably, it omits gravity\u2014so 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 \u201csuper\u00adpartner.\u201d 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\u2019t take too long for physicists to realize that the WIMP was also an excellent dark matter candidate: two problems, one particle.\u00a0<\/p>\n 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.\u00a0<\/p>\n 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, \u201cthe motivation to look for dark matter has not gotten any weaker, right?\u201d\u00a0<\/p>\n Now it\u2019s looking as if those WIMPs\u2014if they exist\u2014may be beyond our current powers of detection. There are a range of difficulties, but the most pervasive is that when you\u2019re 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.<\/p>\n 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. \u201cIt may be that the project doesn\u2019t happen at all,\u201d Lippincott says. \u201cAnd then the US pulling out would have effectively killed it.\u201d\u00a0<\/p>\n 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\u2019s 1050<\/sup>, 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.\u00a0<\/p>\n But many physicists are most interested in the lightest option: the axion.<\/p>\n Like the WIMP, the axion first emerged as a solution to problems with the standard model. In the axion\u2019s 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\u2019t need to\u2014an unanswered question known as the strong CP problem.\u00a0<\/p>\n 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\u2014about 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.)<\/p>\n 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.<\/p>\n 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\u2019t entirely negate background noise. \u201cAt one point, we detected a \u2018message from God\u2019 in our experiment,\u201d Rybka tells me wryly. \u201cWe looked it up in the FCC allocation of spectrum. It was a religious programming station.\u201d\u00a0<\/p>\n So far, particle physicists have combed somewhere between 10% and 20% of the parameter space\u2014the area on a chart showing how massive and interactive dark matter might be\u2014where 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\u2019t address the strong CP problem but could still be dark matter. \u201cMore and more people are thinking about models specifically for dark matter, even without connecting that to any other problem,\u201d says Stefania Gori, a theorist at the University of California, Santa Cruz. \u201cOf course,\u201d she says, \u201cif one can solve more than one problem at once, it\u2019s even nicer.\u201d<\/p>\n As dark matter has continued to go undetected, physicists have dropped some of their theoretical desiderata. Modern candidates don\u2019t need the convenience of WIMPs or the clean simplicity of axions; they need only fulfill the requirements for dark matter.<\/p>\n 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\u2019t have enough heft to produce a clear signal from a collision with atomic nuclei. \u201cOne needed to really come up with new ideas for how one could detect the signals, and then, of course, also new technologies,\u201d says Essig.\u00a0<\/p>\n Researchers have designed novel detectors and installed them in underground labs around the world\u2014often 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 proto\u00adtypes that look for hints in liquid helium, a sensitive superfluid that should throw up a splash when hit by incident dark matter.<\/p>\n 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\u2019s not the quiet space you want if you\u2019re looking for dark matter.\u00a0<\/p>\n This noise has created challenges. In 2020, a surprising number of what physicists call \u201cexcess events\u201d 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 \u201cno.\u201d\u00a0<\/p>\n Background noise can come from anywhere: impurities in silicon-based detectors, materials that have spent too long on Earth\u2019s 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. \u201cIt\u2019s always been true that understanding those backgrounds has been difficult,\u201d says Dan McKinsey, an experimentalist at Lawrence Berkeley National Laboratory. \u201cBut we\u2019ve shifted our regime so quickly that suddenly we don\u2019t understand, as a community, what the key backgrounds are.\u201d<\/p>\n 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.\u00a0<\/p>\n These experiments will help researchers identify the best approaches to use for bigger, more sensitive\u2014and more expensive\u2014detectors. \u201cThere\u2019s still lots of ideas, and it\u2019s still not quite clear what\u2019s going to be the best one to really scale up,\u201d Essig says. For now, if it fits on a table and can plausibly detect dark matter, physicists are willing to try it.<\/p>\n 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<\/a> visible from space. This self-annihilation could also be a strong source of heat\u2014enough to melt a planet\u2019s core. The fact that Earth\u2019s core is solid provides limits for dark matter\u2019s characteristics, and\u00a0measuring the temperatures of planetary\u00a0cores more precisely could set even more stringent constraints.<\/p>\n 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\u2019s moon Ganymede. If dark matter is indeed really heavy\u2014possibly a primordial black hole\u2014it could punch through the surface<\/a> and leave a crater that looks very different from one caused by an asteroid.<\/p>\n Some particle physicists think it would be better to drop all the existing assumptions and refocus. \u201cEverything we know about dark matter has come from its interaction with gravity,\u201d says Caltech\u2019s Zurek. If you concentrate more specifically on that interaction, she says, at least \u201cyou\u2019re guaranteed to learn something.\u201d We know how dark matter behaves on the scale of the observable universe as far down as galaxies. \u201cBelow that, we really know very little about how dark matter collapses under the weight of gravity,\u201d Zurek says. How, for example, does it clump up on the level of a solar system?\u00a0<\/p>\nAstronomical ignorance<\/h3>\n
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Listen closely<\/h3>\n
Quiet revolution<\/h3>\n
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The utmost gravity\u00a0<\/h3>\n
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