A Detector Nearly A Mile Underground Recorded One Particle Interaction. Scientists Still Can't Explain It

Our current understanding of the composition of the universe is that 95% of it is filled with dark matter and dark energy, with dark matter estimated to occupy 27% of the universe. But, the interactions of dark matter with the rest of the physical universe are complicated. We're pretty sure that dark matter exists because it has observable effects. However, dark matter does not absorb, reflect, or emit light (or any other part of the electromagnetic spectrum). So, although we think it's there, we don't know what it's made of. Evidence for the composition of dark matter is actively being sought. One particle found by a detector nearly a mile underground on June 16, 2023 might be that evidence, but it does not fit anything that scientists can yet explain.

The LUX-ZEPLIN detector, which made the observation, is housed nearly one mile below ground at the Sanford Underground Research Facility in South Dakota. It is designed to observe interactions between weakly interacting massive particles (WIMPs) and ordinary matter, in this case liquid xenon. WIMPs are a theoretical form of dark matter and, therefore, can pass right through solid rock. Keeping the detector so far underground is one way to filter out penetrating radiation like cosmic rays or other background noise. Anything that is detected that far underground is more likely to be a WIMP.

Physicists have suspected the existence of dark matter since 1933, when Fritz Zwicky observed that the Coma galaxy was moving too fast to be bound by its visible matter. This concept has been further reinforced by space telescope observations of gravitational lensing, where unseen mass bends light. This evidence was solidified in the 1970s, when astronomers Vera Rubin (namesake of an observatory in Chile), Kent Ford, and Mort Roberts used spectrographic and radio data to discover that outer stars orbit galaxies just as fast as inner ones. Evidence for the effects of dark matter was confirmed. But still, evidence of its composition remains elusive.

The 2023 signal could be a WIMP. Its discoverers have investigated and ruled out many other possible sources of the observed signal. If verified, it would be the first dark matter particle ever detected. However, a single interaction falls short of the five-sigma particle discovery threshold, so the LUX-ZEPLIN researchers will need more observations of similar interactions before any theories can be updated. The 2023 observation provides a hint that particle physicists may be on the right track about the nature of the universe. But it could go the other way too, and prove to be just another false alarm. 

The Nancy Grace Roman Space Telescope, launched on August 30, 2026, is intended to support the search for dark matter. Additional observations of WIMPs and other evidence for dark matter may follow in the future. The mysteries of spacetime may soon reveal themselves, but for now, the LUX-ZEPLIN particle remains unexplainable.