Potential dark matter detection sparks hope in search for elusive substance

By Jacopo Prisco, CNN

(CNN) — An international team of scientists says it has detected an intriguing signal that could hint at evidence of dark matter, offering a clue that might bring the mystery of the elusive substance one step closer to a solution.

Dark matter makes up about 85% of all the matter in the universe and is about five times as abundant as ordinary matter, which makes up stars, planets and everything else that scientists can see. Dark matter is invisible because it does not absorb or reflect light, but it does interact with regular matter, and its gravitational effects are needed to explain the structure of the universe.

For nearly half a century — since American astronomers Vera Rubin and W. Kent Ford provided some of the strongest evidence for dark matter’s existence — scientists have tried to identify the unseen substance, more recently using sophisticated devices designed to detect potential candidates.

One of these devices is the LUX-ZEPLIN, or LZ, experiment — a detector containing 7 active metric tons of liquid xenon that’s in a former gold mine nearly a mile (about 1.5 kilometers) below Earth’s surface at the Sanford Underground Research Facility in South Dakota. The detector registered an unusual particle interaction in June 2023 that generated a flash of light and cautious excitement.

The LZ collaboration is an international group of 250 scientists and engineers from 39 institutions. After months of analysis, the team estimates only a 0.5% chance that a known source of interference caused the event — making this signal the most compelling hint of dark matter the instrument has ever recorded.

However, in scientific terms, claiming a discovery requires a much higher degree of confidence. “One event, by itself, is not enough,” Alvine Kamaha, an assistant professor of physics at the University of California, Los Angeles, said in an email.

“We need to see whether additional events appear as we collect more data and whether the statistical significance of the observation increases,” said Kamaha, a member of the LZ collaboration who helped build the LZ detector.

Sam Eriksen, a senior research associate of physics at the University of Bristol and member of the LZ collaboration, presented the findings September 1 at the TeV Particle Astrophysics 2026 conference in Japan, and the team has submitted a study for publication in the scientific journal Physical Review Letters.

The researchers are already working on further analysis that could potentially increase the statistical significance of the event. The threshold to claim a discovery in particle physics is known as 5-sigma, which means about a 1 in 3.5 million chance of a statistical fluke rather than a real hint of dark matter. The analysis is currently at 2.6-sigma, or a 1 in 200 chance of a fluke.

A definitive detection of dark matter would be “a major breakthrough,” Kamaha said. “We know that dark matter plays a fundamental role in the formation of galaxies and the large-scale structure of the universe, but we still do not know what it actually is,” she said. “It’s exciting because it would open an entirely new area of particle physics.”

Planting fake dark matter signals

There are several candidates for what dark matter could be, including primordial black holes or an undiscovered particle. Detectors such as the LZ experiment search for a class of hypothetical particles called weakly interacting massive particles, or WIMPs.

If they exist, WIMPs pass through regular matter without interacting with it, and vast numbers of them could pass unnoticed through a human body every second. On rare occasions, however, one such particle could collide with an atomic nucleus and produce a tiny recoil — precisely the kind of event the LZ experiment is designed to detect.

The detector uses highly purified liquid xenon. Researchers selected this element because its atoms have heavy nuclei that make it a particularly sensitive target for WIMPs. Collisions in xenon produce signals the detector can easily measure.

The LZ experiment is located deep underground and equipped with protective layers to shield it from cosmic rays and other sources of radiation, which could produce signals similar to dark matter. However, this background noise can only be reduced, not eliminated.

“You’re always going to be in a situation where it’s possible that events occurring in your detector are due to more conventional mechanisms,” said Rick Gaitskell, Hazard Professor of Physics at Brown University in Providence, Rhode Island, and the spokesperson for the LZ experiment.

Potential dark matter collisions are believed to be extremely rare. “Our understanding is that dark matter is so weakly interacting with conventional material,” Gaitskell added, “that even in a detector of the scale of the LZ experiment we need to look for periods of months or years per single interaction.”

The LZ collaboration spotted the 2023 event through an analysis of 220 days of data, collected between March 2023 and April 2024. The researchers are now working through a more recent dataset spanning 700 days, Gaitskell said, hoping it contains further collisions that could help determine whether the 2023 event was a dark matter interaction.

For this larger analysis, the team is also introducing techniques to avoid unconscious bias, for example by inserting “synthetic events” in the data. These look like genuine dark matter interactions to the analysis team and are only removed after the analysis is completed.

“A possible detection brings a lot of tension between the excitement of the experiment working the way we imagined it would, with the worry that we could make mistakes, or just simply be fooled by something rare and new or coincidental happening in the detector,” said Kimberly Palladino, a professor of physics at England’s University of Oxford and a member of the LZ collaboration, in an email. “It’s a little like having a crush on someone as a teenager where you tell yourself to act cool, but have the tendency to over-interpret every little gesture they make.”

However, Palladino warned, history is also littered with experiments that have seen one or two unexplained events that are never fully understood. If the new LZ data contains new potential dark matter collisions, similar experiments also designed to detect dark matter, such as the XENONnT in Italy and the PandaX-4T in China, could provide an independent test of the results.

Finding dark matter would bring scientists a big step closer to understanding what the universe is made of and how it evolved from the big bang to present day.

“But a lot more scientists will need to study dark matter to understand its properties in a variety of different experiments, and then using that information, run astrophysical simulations of our universe,” Palladino added. “There are many theories about what dark matter can be, and there may be multiple types of dark matter.”

A lot still to do and learn

The LZ experiment’s potential detection of dark matter is intriguing but needs confirmation, according to Tim M.P. Tait, a professor in the department of physics and astronomy at the University of California, Irvine, who’s not part of the LZ collaboration.

“Only time can tell whether they will see more events as the detector accumulates more data, or if this will turn out to be a temporary statistical fluke,” Tait added in an email. He noted that the LZ event occurred at a much higher energy than most models expect for WIMPs, meaning dark matter could “turn out to be more weird and wonderful than even we had originally imagined.”

Tracy Slatyer, a professor of physics at the Massachusetts Institute of Technology, agrees that further data analysis is needed to understand whether the LZ event is dark matter. “If this is dark matter,” Slatyer, who is not involved with the LZ collaboration, added in an email, “the fact that the event is at quite a high energy, without accompanying events at lower energy, is very interesting.”

The energy measured in the LZ event indicates how much the xenon nucleus recoiled due to the potential interaction with the dark matter. A high-energy dark matter particle, Slatyer explained, would already reveal quite a bit about the nature of dark matter and how it interacts with ordinary particles. Other experiments could potentially help pin down its properties further.

“There would be a lot still to do and learn, but if this really is a dark matter signal,” she said, “this could be a key that unlocks a great deal of information about new physics, as well as giving us a new way to measure the behavior of dark matter in the neighborhood of the Earth and possibly more broadly through the cosmos.”

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