Dark Matter Hunters Spot a Signal They Cannot Yet Explain

Scientists on the LUX-ZEPLIN (LZ) dark matter experiment have recorded a single particle interaction that they cannot yet explain using any known background source. It is not strong enough to count as a discovery, but researchers call it the most compelling potential dark matter signal LZ has reported so far.

LZ is an international collaboration of 250 scientists and engineers from 39 institutions, run by the U.S. Department of Energy's Lawrence Berkeley National Laboratory. The detector sits nearly a mile underground at the Sanford Underground Research Facility in South Dakota and uses 10 tonnes of ultra-pure liquid xenon to hunt for WIMPs, or weakly interacting massive particles, one of the leading candidates proposed to explain dark matter. The results were presented at the 2026 TeV Particle Astrophysics conference in Japan and will be submitted to Physical Review Letters.

"We're very intrigued to see this event in the data, in the region where we expect dark matter to show up and the competing backgrounds are very low," said Rick Gaitskell, a professor at Brown University and LZ's spokesperson. "We are not claiming to have seen dark matter. But we have seen something interesting that we want to share with the scientific community." The team analyzed 220 live days of data gathered between March 2023 and April 2024, this time widening its search beyond the simplest WIMP interaction signatures to include ones that deposit more energy in the detector.

If dark matter really did produce the signal, the responsible WIMP would likely have a mass of at least 200 GeV/c2 — more than 200 times the mass of a proton — pointing to a type of interaction beyond the simplest models usually considered. Particle physics generally requires a result to reach "5-sigma" statistical significance to count as a discovery; the new finding currently sits at 2.6 sigma, meaning roughly a 0.5% chance it comes from a known background source.

"Outlier events in the data are not unexpected, but they usually stand out as a background of some kind when you look at them deeper," said Aaron Manalaysay, a physicist at Berkeley Lab and chair of LZ's Institutional Board. "This is the first example in any experiment I've worked on of an outlier that appears valid in every way."