Dark Matter Still Has No Confirmed Particle After 40 Years

Dark matter is thought to make up 85 percent of the total mass in the universe, according to the standard cosmological model. Its existence is inferred from the gravitational pull it exerts on galaxies, on clusters of galaxies, and on the cosmic microwave background, the faint afterglow of the Big Bang. For more than 80 years since the hypothesis was first proposed, the case for dark matter has only grown stronger.

The latest results from the LUX-ZEPLIN experiment, the most sensitive dark matter detector ever built, were released on December 8, 2025, and they deepen the puzzle. Once again, the search found nothing. As the experiment enters what researchers call the neutrino fog, the WIMP hypothesis is being squeezed from every direction at once, and the field is widening the hunt.

The Gravitational Case for Dark Matter

The evidence that something unseen holds the universe together is broad and consistent, and it all comes from gravity. Galaxies in clusters move too fast for the visible matter alone to hold them, a fact first measured by the Swiss-American astronomer Fritz Zwicky in 1933 when he applied the virial theorem to the Coma Cluster. Stars at the edges of spiral galaxies orbit so quickly that, by the laws of motion worked out three centuries ago, they should fly off into intergalactic space, unless a large halo of unseen mass is keeping them in.

This pattern repeats at every scale astronomers can measure. Galaxy clusters bend the light of more distant objects behind them through gravitational lensing, an effect that requires more mass than the visible galaxies and hot gas can supply. The cosmic microwave background, mapped in detail in Planck’s full inventory of the universe’s ingredients, carries temperature fluctuations whose pattern fits only if the early universe contained far more matter than the ordinary kind. In a famous pair of colliding galaxy clusters known as the Bullet Cluster, the bulk of the mass sits apart from the visible gas, as though an invisible substance had sailed through the collision untouched.

Forty Years Underground, and Nothing Yet

To turn gravitational inference into proof, physicists need to catch a dark matter particle interacting with ordinary matter in a detector. The dominant strategy has been to build exquisitely sensitive instruments, bury them deep underground to screen out the constant rain of cosmic rays, and wait. The leading experiments use tanks of liquid xenon, a dense noble gas that flashes with light whenever a particle deposits energy inside it.

These detectors have grown from kilograms of material to multi-tonne instruments. The current state of the art sits nearly a mile underground at the Sanford Underground Research Facility in South Dakota, in a cavern once used for a Nobel-winning neutrino experiment. The LUX-ZEPLIN experiment, or LZ, holds 7 tonnes of liquid xenon in a cylindrical time projection chamber wrapped in liquid scintillator and water shielding. It is one of three flagship detectors racing to find the same signal, alongside XENONnT in Italy’s Gran Sasso laboratory and PandaX-4T in China.

The result has been the same at every iteration. No signal.

The cumulative effect is striking. In results first reported in 2022 and tightened repeatedly since, the LZ collaboration has set successive world-leading limits on how strongly WIMPs can interact with ordinary nuclei. As of late August 2024 the experiment had worked for 280 days without finding evidence of dark matter, tightening the limits on its properties. The pattern is now familiar enough that physicists describe the hunt not as a series of attempts but as a continuous squeeze.

By the numbers, sourced from the LZ collaboration and the standard cosmological model:

• 85% of all matter in the universe is dark matter
• 7 tonnes of liquid xenon sit at the heart of the LZ detector
• About 1,500 metres of rock shield the LZ detector from cosmic rays
• 280 days of LZ exposure had produced no signal as of late August 2024
• 417 live days of data went into the December 8, 2025 result

The LZ December 2025 Milestone

The LZ December 2025 result, announced on December 8, was the largest single dataset ever released by a dark matter detector, and it both closed off more territory for the WIMP hypothesis and opened a new window on a different particle. The collaboration analyzed 417 live days of data, collected between March 2023 and April 2025, and probed a mass range between 3 and 9 GeV/c², roughly three to nine times the mass of a proton, the first time LZ has looked for WIMPs below 9 GeV/c². The result was no WIMP signal, and the tightest constraints yet on low-mass dark matter interactions.

While we don’t see any direct evidence of dark matter events at this time, our detector continues to perform well, and we will continue to push its sensitivity to explore new models of dark matter. As with so much of science, it can take many deliberate steps before you reach a discovery, and it’s remarkable to realize how far we’ve come. Our latest detector is over 3 million times more sensitive than the ones I used when I started working in this field.

Rick Gaitskell, a physics professor at Brown University and spokesperson for the LUX-ZEPLIN collaboration, spoke at a webinar hosted at the Sanford Underground Research Facility on the day of the announcement. The detector itself is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, and the wider collaboration includes 250 scientists and engineers from 37 institutions. LZ is scheduled to collect over 1,000 days of live search data by 2028, more than doubling the exposure that produced the December 2025 results. Brown University research teams contributed the photomultiplier tube arrays and the calibration systems that made the run possible.

The same dataset produced a different first. LZ picked up the clearest signal yet from boron-8 solar neutrinos interacting with xenon nuclei, a process called coherent elastic neutrino-nucleus scattering that was first observed only in 2017. The collaboration measured the signal at 4.5 sigma, a confidence level physicists treat as evidence, even if not yet as a discovery. Two earlier dark matter detectors, PandaX-4T and XENONnT, had seen hints of the same effect at 2.64 and 2.73 sigma respectively, shy of the conventional threshold. The LZ measurement is independent confirmation that neutrino detectors have crossed a new sensitivity bar, and it gives solar physicists a fresh tool for measuring the Sun’s nuclear furnace.

From WIMPs to a Wider Field

The WIMP hypothesis is the natural target of these detectors because it fits neatly with the rest of particle physics. WIMPs, short for weakly interacting massive particles, are predicted by supersymmetry, a theoretical framework that pairs every known particle with a heavier partner, and they would have been produced in the right abundance in the early universe to account for all the dark matter astronomers see. They are heavy, by subatomic standards, and they barely interact with ordinary matter, which is what makes them so hard to catch.

For three decades the WIMP was the front-runner, and the world’s dark matter program was built around it. The Large Hadron Collider at CERN has spent years searching for the supersymmetric particles the WIMP framework implies, without success. The underground detectors have pushed interaction limits down by orders of magnitude across the standard GeV-to-TeV mass range, as the catalog of liquid-xenon detector results tracks. With WIMPs squeezed into smaller and smaller corners of parameter space, researchers have widened the hunt to a longer list of candidates.

The current shortlist of leading candidates:

• WIMPs: heavy, weakly interacting particles predicted by supersymmetry, the focus of the underground program
• Axions: extremely light particles that interact with photons inside strong magnetic fields, the target of dedicated microwave-cavity experiments
• Primordial black holes: exotic compact objects that may have formed in the early universe and could make up some or all of the dark matter today
• Modified gravity: theories such as MOND that replace dark matter with revisions to the laws of gravity, pursued by a minority of astrophysicists

The Neutrino Fog and What Comes Next

The latest LZ results mark a transition. With its detector now sensitive enough to register solar neutrinos through coherent elastic neutrino-nucleus scattering, the experiment has officially entered what researchers call the neutrino fog, a regime in which neutrino signals begin to compete with potential dark matter signals at low masses. Ann Wang, an associate staff scientist at SLAC National Accelerator Laboratory and a co-lead of the analysis, drew the line: the experiment has entered the fog for low-mass dark matter, while heavier candidates remain unaffected.

The fog is not an accident. It is the inevitable consequence of pushing detectors to sensitivities where neutrinos, which stream through every kilogram of matter on Earth every second, finally become visible. As future detectors chase WIMP signals at lower and lower masses, they will have to find ways to subtract that neutrino background, or to use it as a calibration tool. The CERN Courier, surveying the field, has framed the goal of next-generation detectors as exploring the WIMP parameter space down to the point where the WIMP signal becomes indistinguishable from the background of coherent neutrino-nucleus scattering events.

The wider field is responding in three ways. Liquid-xenon detectors are scaling up, with the next-generation DARWIN project planned for Italy’s Gran Sasso laboratory and designed to reach sensitivities that would either detect WIMPs or rule out the entire accessible parameter space. Dedicated axion experiments continue to widen their scan of the axion mass range. And the cosmological side is sharpening its own picture, with surveys of the cosmic microwave background and large-scale structure setting tighter constraints on what any dark matter candidate must explain.

A lingering alternative treats dark matter as a sign that gravity itself needs revising. Modified Newtonian dynamics, MOND, and related theories can reproduce the rotation curves of spiral galaxies without any hidden mass, and they remain popular with a minority of astrophysicists. But no modified gravity theory has yet explained the cosmic microwave background and the separated mass in the Bullet Cluster simultaneously, which is why a particle remains the leading bet for most researchers. The combined data from the COSINE-100 and ANAIS-112 experiments, which used sodium-iodide detectors identical in design to the long-running DAMA/LIBRA experiment, ruled out a dark matter origin for DAMA’s annual modulation signal at high confidence in a Physical Review Letters study that effectively retired the long-running claim. The detection that survived for nearly three decades no longer carries weight.

How the three flagship underground detectors compare on the latest run:

Frequently Asked Questions

What is dark matter made of?

No one knows. The leading particle candidates are weakly interacting massive particles (WIMPs), extremely light particles called axions, and primordial black holes formed in the early universe. A smaller group of researchers still pursue modified gravity theories such as MOND, which explain the same observations without any new kind of matter at all.

Why hasn’t anyone detected dark matter yet?

The most sensitive direct-detection experiments have spent decades refining their sensitivity by orders of magnitude, and the LZ collaboration alone has accumulated hundreds of days of exposure without finding a WIMP signal. The leading experiments work by burying large tanks of liquid xenon deep underground to shield out cosmic rays and waiting for a dark matter particle to recoil off a xenon nucleus. The detection would be rare, faint, and easy to miss against any background noise.

What is the LZ experiment?

LZ stands for LUX-ZEPLIN, a direct-detection dark matter experiment operated by a collaboration of 250 scientists and engineers from 37 institutions. It uses 7 tonnes of ultra-pure liquid xenon as its target, sits about 1,500 metres below ground at the Sanford Underground Research Facility in South Dakota, and is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.

What is the neutrino fog?

It is the point at which neutrino signals in a dark matter detector begin to compete with the faint recoil signals that would come from a low-mass WIMP. As detectors become more sensitive, neutrinos from the Sun and other sources become visible too, and their interactions can mimic the pattern expected from a dark matter particle. The December 2025 LZ results were the first from a dark matter experiment to clearly measure the boron-8 solar neutrino signal at 4.5 sigma.

How much of the universe is dark matter?

According to measurements from the European Space Agency’s Planck mission, dark matter makes up about 26.8 percent of the universe’s mass-energy content, ordinary matter makes up about 4.9 percent, and dark energy accounts for the remaining 68.3 percent. Because dark energy is not matter, dark matter is about 85 percent of all the matter in the universe.