Have we finally found dark matter?

On June 16, 2023, an unusual flash of light was detected in a dark, sealed tank holding extremely cold liquid. This occurred 39 seconds after 3:22 PM and one mile beneath the Black Hills of South Dakota. The light flashed within the LUX-ZEPLIN (LZ) experiment, a project searching for dark matter. The tank is a key component of the LZ experiment, which has been one of several attempts by physicists to capture and examine dark matter, a mysterious substance believed to be present throughout the universe but seemingly undetectable to date.

Scientists have been searching for dark matter for 40 years, placing detectors in the quietest and most secluded places on Earth, where the elusive substance's presence might be detected. However, every observed flash of light or surge of electricity in the past had more mundane explanations. This particular flash from three years ago, though, appeared to indicate a violent collision between an unknown particle and an atom's nucleus in the detector.

This event did not resemble any of the common sources of flashes that the LZ collaboration's hundreds of scientists had spent their careers trying to eliminate. Nonetheless, the collision was also atypical of the dark matter signal they were seeking. This news was shared at a presentation at the TeVPA conference in Japan, with an accompanying paper submitted to Physical Review Letters.

The chances of this flash being a statistical error were estimated at around half a percent, which is still several times higher than the threshold normally used in particle physics experiments to declare a discovery. The universe we observe from space could not be comprehended without dark matter. The matter we can see is moving too quickly, forming too quickly into clumps, and is too dense to easily explain.

The simplest explanation is that there is about five times more dark matter than the visible matter, yet we have no idea what it is made of. Detectors like LZ operate on the assumption that dark matter is made of weakly interacting massive particles (WIMPs), hypothetical particles that usually pass through ordinary matter like ghosts but occasionally interact with it by colliding with an atom.

LZ and its competitors are rushing to pack as many atoms as possible into the quietest places they can find to capture these collisions. However, background radioactivity can still interfere with the underground silence, with this activity breaking through LZ's mile of protective bedrock or even seeping out of the materials lining the detector's walls.

But physicists can usually distinguish many of these events from their best guess at what dark matter's signals should look like and strive to reduce these incidents as much as possible. The single unexplained event that occurred carries a significant amount of statistical significance, demonstrating how sensitive the technology has become.

"We made our meter so that the needle would jump when it saw an event like this," says Knut Morén, a physicist at the University of Zurich and a member of the LZ collaboration. "Everything is working as intended." However, the collision from June 2023 meets most of the team's WIMP expectations, except that it is far too energetic.

For LZ to detect such a high-energy WIMP event, most standard theories suggest that it should have already observed dozens or hundreds of gentler ones, which it has not. Therefore, if what LZ saw is a WIMP, this particle must be exceptionally unusual—interacting differently with regular matter or having been accelerated to a higher speed.

"Given that we don't know what the dark matter is, it doesn't strike me as strange that it might not be exactly the thing we expect," says Hugh Lippincott, an associate professor at the University of California, Santa Barbara, and a member of the LZ collaboration. The collaboration itself advises caution, stating that they should not claim specific things about dark matter or new physics based on one event.

They also emphasize that they analyzed the data without "blinding" themselves to it, an approach to avoid bias that is considered the gold standard for particle physics. Fortunately, more answers should follow in the near future, based on a larger set of data that has been properly blinded. "This was 200 days of data. We actually have over 700 days blinded," says Richard Gaitskell, a professor at Brown University and co-spokesperson of LZ.

"So that puts us in a strong position to study this." Those data will either reveal the nature of dark matter or further illuminate the mystery.