A detector buried a mile underground in South Dakota has recorded an anomalous interaction that scientists believe is the first evidence hinting at dark matter. While this single incident doesn’t prove the long-awaited discovery, researchers are excited to confront tangible data that cannot be explained with traditional processes.
Dark matter is a name given to an invisible, undetected substance that makes up 85% of the universe’s matter. Its existence was inferred from gravitational effects that provide possible explanations for several unexplained phenomena. The most well-known example is the reason galaxies are held together while they rotate unexpectedly fast. While decades have been spent determining what dark matter consists of, no definite conclusions or detections have been made. The most promising conclusion describes them as a new set of weakly interacting massive particles, or WIMPs.
The Sanford Underground Research Facility in Lead, South Dakota, conducted the LUX-ZEPLIN, or LZ, experiment. Underneath the Black Hills, the facility stores tonnes of ultrapure liquid xenon to search for rare particle collisions. Additional layers of water and detectors were installed around the xenon chamber to separate ordinary interactions from cosmic radiation and extract meaningful data. The team has been recording data since March 2023, but it was in September 2026 that they finally found a number that stood out.
As Earth moves through the Milky Way, scientists assume the planet passes through a halo of dark matter scattered across the universe. If the hypothesis of WIMPs is valid, there is always a chance of them interacting with the nucleus inside the detector. Xenon is useful as its heavy nuclei provide a reliable target for possible interactions. When the WIMPs interact with the nuclei, it recoils and deposits energy in the liquid. This brief change in energy level creates a flash that can be detected by sensitive photomultiplier tubes in the facility. By analyzing this data, scientists can specify the location and energy transfer of the interaction.
This specific event detected was an anomalous interaction. The xenon nucleus recoiled with an estimated energy of around 248 keV in a region where researchers estimated a small number of background events. Even though scientists can not yet fully determine if the collision was caused by dark matter, it has, so far, not been explained by any classical explanations. The research team determined the significance as 2.6 sigma after accounting for the range of models searched. While it is well below the 5 sigma threshold to claim a discovery, it is worth closer attention.
Fermilab experimental physicist Dr. Max Fieg described the detection as “a meaningful milestone for particle physics.” Even though he agrees with the uncertainty of the event, he is hopeful for further research.
“It helps particle physics to gain the attention it needs,” he said, adding that the research may be the “door for physics beyond the Standard Model.”
The uncertainty of the LZ event reflects how major discoveries in particle physics develop over time. The Higgs Boson, similar to dark matter, has previously been theorized but not detected. It was one anomalous detection that proved its existence. While the LZ experiment could be another speck of data, its importance will depend on whether future observations reveal the same pattern.