Deep beneath the Black Hills of South Dakota, US, in an old gold mine, a tank of liquid xenon has been used to produce an unusual particle reaction that may be a link to “dark matter”.
Scientists conducting an experiment there say they have found a single “unexplained particle interaction” that resembles a dark matter particle, though researchers have not called this a confirmed detection.
“Dark matter” and “dark energy” are substances whose presence humans have detected, but have yet to actually observe. This is the closest scientists have come to possibly uncovering the universe’s most elusive form of matter.
Here’s what we know.
What is dark matter?
Dark matter is an invisible substance that is believed to make up roughly 27 percent of all matter in the universe. Similarly, dark energy, thought to make up 68 percent of everything in the universe, is an invisible force that pushes galaxies apart and speeds up the expansion of the universe.
Ordinary matter, which is everything we can see and touch, accounts for only 5 percent of everything that exists.
Scientists found proof that this unseen matter exists in the universe by observing its gravitational pull, which affects how galaxies spin and how light bends around large clusters of it.
What it is made of, and what it looks like, has remained a mystery to scientists since its discovery nearly a century ago, as it cannot be detected directly because it does not reflect light or electromagnetic radiation.
The first clue to the existence of dark matter was discovered in the 1930s, when Swiss astronomer Fritz Zwicky noticed that the galaxies in the Coma Cluster were moving too fast for the cluster’s visible mass to hold them in place, suggesting something else had to be applying gravity. Similar observations were made over the following decades – all signs pointing to the presence of invisible matter, scientists say.
On Sunday, NASA, the US space agency, announced that it was launching a $4bn project, the Nancy Grace Roman Space Telescope, which will investigate both dark matter and dark energy.
What have scientists found in the latest experiment?
Scientists say they have found a single unexplained particle interaction that resembles a dark matter particle, though they said this is not the same as directly detecting dark matter itself.
The LUX-ZEPLIN (LZ) experiment was run by an international team of 250 scientists and engineers from 38 institutions. At its centre is an underground tank at the Sanford Underground Research Facility (SURF) in the state of South Dakota, in the United States, which holds ultra-pure liquid xenon, surrounded by hundreds of light sensors.
The theory that the experiment was testing is this: if a hypothetical dark matter particle called a WIMP (weakly interacting massive particle) reacts with the nucleus of a xenon atom, it would produce two flashes of light at a specific energy level.
After reviewing 220 days of data collected between March 2023 and April 2024, the LZ team discovered one such event on June 16, 2023.
Sam Eriksen, the lead of the study and a senior research associate at the University of Bristol, presented the findings on Tuesday at the 2026 TeV Particle Astrophysics conference in Japan.
“We understand our detectors and the backgrounds so well,” Eriksen explained in a press release, “that even a single outstanding event, like the one we found, is important”.
Rick Gaitskell, the LZ spokesperson and a physicist at Brown University, said scientists “don’t want to get ahead” of themselves.
“We are not claiming to have seen dark matter,” he added.
How significant is this?
Physicists are calling this the most compelling signal LZ has produced so far, and it could be a signpost to the first real evidence of a particle that physics as we currently understand it has not been able to explain.
Theresa Fruth, a physicist at the University of Sydney who also worked on the study, told ABC News that the event is exciting because it has been re-tested several times and has held up. “This event just won’t go away even after many, many checks.”
