Scientists Hunting Dark Matter Found Something Strange

A team of scientists running a special experiment deep underground in South Dakota may have spotted something never seen before. Researchers working on the LZ Dark Matter Experiment detected a single, unusual signal that doesn't match any known particle. The finding was shared at a science conference in Japan on Tuesday and has physicists around the world paying close attention.

Dark matter is a mysterious substance that scientists believe makes up about 85 percent of all matter in the universe. Even though it is everywhere, we can't see it because it doesn't give off, absorb, or reflect any light. Scientists know it exists because of the way its gravity affects galaxies and stars, but no one has ever directly detected it.

One popular idea is that dark matter is made of particles called WIMPs, which stands for weakly interacting massive particles. WIMPs are thought to be heavy compared to most subatomic particles, and they rarely bump into regular matter. When they do, theories say they should create a small burst of energy that a sensitive detector could pick up.

The LZ detector is built to catch exactly that kind of signal. It is a large tank holding more than seven tons of liquid xenon, stored one mile underground at the Sanford Underground Research Facility. The underground location helps block radiation from space that would otherwise flood the detector with millions of unwanted signals every second.

Even with all those protections, scientists still have to work hard to filter out noise from other particles. Water surrounds the detector to block tiny particles called neutrons, and the xenon is carefully purified to remove anything radioactive. 'It's kind of like looking for a needle in a haystack,' said Alvine Kamaha, a physicist at the University of California, Los Angeles, who helped build the detector.

Scientists spent 220 days collecting data and sorting through dozens of particle events recorded each day. They first looked for signals that matched the simplest ideas about what a WIMP would do, but found nothing. When they widened their search to include more complex theories, they found one single event worth examining closely.

That event happened on June 16, 2023, when a particle entered the detector and bumped into the nucleus of a xenon atom. It created a flash of light and a stream of electric charge, which is exactly the kind of signal scientists were hoping to see. But just one event is not enough to be sure of anything yet.

Physicists put the odds of the signal being a random mistake at about 1 in 400. In science, that sounds unlikely, but it is not nearly enough to claim a real discovery. The gold standard for declaring a discovery requires much stronger confidence — about 3,100 times stronger, in fact.

Richard Gaitskell, the spokesperson for the LZ team, said the event is exciting but that it could still turn out to be ordinary matter behaving in a way scientists don't fully understand. He said the team wanted to share the results so other scientists could weigh in. 'After doing so much work internally, we felt ready to talk to the rest of the world about the results,' he said.

The LZ detector has already collected nearly four times more data since this study ended, and scientists are now working through all of it. Two other experiments — one in Italy and one in China — are also searching for dark matter using similar liquid xenon detectors. Scientists hope that comparing results from all three experiments will help solve the mystery.

Katherine Freese, a physicist at the University of Texas at Austin who was not part of the LZ team, called the result thrilling. She praised the techniques the team used to study the data as 'phenomenal.' For now, though, the search for dark matter goes on — and scientists say that is exactly how good science is supposed to work.