Dark matter detector records most compelling hint yet, physicists say

Dark matter, an elusive substance that permeates the universe, has flummoxed scientists for decades.

But a single flash of energy spotted by the world's most sensitive detector of its kind is the "most compelling hint reported to date", according to an international team of physicists.

The team made the discovery, which is yet to be confirmed, using the LUX-ZEPLIN detector in the US.

They presented their findings today at the 2026 TeV Particle Astrophysics conference in Japan, and in a paper which is yet to be peer reviewed on the preprint server arXiv.

Theresa Fruth, one of the Australian physicists on the team, said the result was "very exciting".

"This event just won't go away even after many, many checks," Dr Fruth, from the University of Sydney, said.

But, she said, more detections were needed to confirm the result.

Nicole Bell, a theoretical physicist at the University of Melbourne who was not involved in the research, said it was an "interesting observation" but it was "very early days".

"If it's really dark matter, we'll see more events soon, and that would be a huge breakthrough," Professor Bell said.

What is dark matter?

Scientists estimate that 85 per cent of the observable universe is dark matter.

They know it exists by watching the way galaxies move, but no-one has observed it directly.

"What we do know is that there must be a lot more matter out there in the universe than what we know about."

One leading theory suggests dark matter might be slow-moving particles, bigger than the components of atoms, called weakly interacting massive particles (WIMPs).

What did the team find?

Built in an old gold mine almost 1.5 kilometres underground, LUX-ZEPLIN is the most sensitive of the recent generation of dark matter detectors.

It uses a huge vat of super-cooled liquid xenon and 494 light sensors to try to detect tiny collisions between WIMPS and normal particles.

Until now, the LUX-ZEPLIN detector had not detected anything that looked like dark matter.

It is not alone, no other detectors have provided concrete evidence for dark matter, and most have found nothing at all.

Only one, the DAMA/LIBRA experiment in Italy, has found an annual signal which they attribute to dark matter, but no other detector has been able to confirm the results after years of attempts.

Professor Bell said the signal detected by LUX-ZEPLIN was "very different", but perhaps easier to explain as dark matter.

The single event, which was detected on June 16, 2023, was part of a 220 day run of the detector.

Jaden Newstead, a physicist at the University of Melbourne who also was not involved in the research, said physicists would need to rework some of their theories about dark matter if the result was confirmed.

"It's not how we would expect dark matter to have appeared," Dr Newstead said.

"This was a special analysis that was looking at higher energies."

Originally, many detectors searching for WIMPs looked at a much lower energy band, as that is what was predicted by theoretical physics.

This detection had an energy range of 270 keV, meaning the WIMP was even more massive than the researchers assumed.

The team looked for "background" interference that might have caused the result, such as neutrons and neutrinos, but found "none could be identified as a likely explanation".

The team reached what is known as 2.6 sigma, a 0.5 per cent chance that the event could be explained by known backgrounds.

This is still below a 5 sigma threshold needed to confirm a discovery.

Dr Fruth said it was still possible that some "ultra-rare" background could have caused the detection.

When will we know if we've seen dark matter?

The LUX-ZEPLIN experiment will continue to collect data until at least 2028.

The team will use this data to either build their confidence in the finding or break it down.

Even if it does not pan out, Dr Fruth said the result would still be interesting.

"It's certainly something new and exciting to investigate further."

Similar dark matter detectors, like the XENON project in Italy and the PandaX project in China, could help check their results.

Dr Fruth said checking in with other scientists was one of the reasons they have released the data now instead of waiting for it to be published in a peer-reviewed paper.

Dr Newstead said it would likely take months for other experiments to check their data and confirm or refute whether the dark matter detection was real.

"It's an exciting hint," Dr Newstead said.

"Whenever the experimentalists are confused about something, it's always exciting for theorists."