Bucknell Professor Abby Kopec Part of Breakthrough Dark Matter Research

Bucknell University Professor Abby Kopec, physics & astronomy, is part of an international team of scientists that has reached a new milestone in the global search for dark matter — the mysterious substance that makes up most of the matter in the universe but has never been directly observed.

The team, made up of more than 200 researchers from about 30 institutions worldwide, is conducting its work at the XENONnT experiment, located a mile underground at Italy's Gran Sasso National Laboratory. Their latest findings were published March 20 in the journal Physical Review Letters in a paper titled "First Search for Light Dark Matter in the Neutrino Fog with XENONnT."

Kopec says the research pushes the boundaries of what current technology can detect — and could ultimately lead to the first direct evidence of a dark matter particle.

"There's overwhelming evidence that something invisible is shaping how galaxies move and evolve," Kopec says. "We call it dark matter. It doesn't emit or absorb light, but it has gravity, and we know it must be there."

Dark matter is believed to make up about 27% of the universe, while ordinary matter — the matter that makes up stars, planets and people — accounts for less than 5%. The remaining 68% is dark energy, another mysterious force that drives the universe’s expansion.

The XENONnT experiment is designed to detect possible interactions between dark matter particles and atoms. To do that, the team uses a giant tank filled with nearly six tons of ultra-pure liquid xenon. When a particle passes through and hits a xenon atom, it creates a tiny flash of light, which is picked up by highly sensitive detectors.

"This setup allows us to search for WIMPs — weakly interacting massive particles — which are one of the leading candidates for what dark matter might be made of," Kopec says. "We've now reached a level of sensitivity that allows us to search for lighter forms of dark matter that were previously undetectable."

The challenge, Kopec says, is that the more sensitive the detector becomes, the more it starts to pick up other signals, like neutrinos from the sun. Neutrinos are tiny, nearly massless particles that constantly pass through Earth and rarely interact with anything. Their presence in the detector, sometimes called the "neutrino fog," can make it harder to identify true dark matter interactions.

"This summer, our team confirmed that we are now detecting solar neutrinos in the experiment," Kopec says. "That's actually great news — it tells us our detector is working exactly the way it should and is reaching the level of precision we need to start seeing possible dark matter signals.

Although the team hasn’t yet identified a dark matter particle, the results are a critical step forward. By lowering the energy threshold of the detector, the researchers were able to expand their search into new territory, where they expect lighter dark matter particles might exist.

"Everything we know to have mass has a particle associated with it," Kopec says. "Dark matter should be no different. We just haven't found its particle yet."

Some scientists believe that dark matter may have originated early in the universe, possibly splitting off from ordinary matter through a fundamental force known as the weak force. This force, which is involved in radioactive decay and other nuclear reactions, could hold the key to explaining dark matter's elusive properties.

"There are still a lot of places to look," Kopec adds. "Our current detector has helped us reach a very important threshold. But to go further, we’ll need even larger detectors — maybe 100 times the size of what we're using now."

Until then, the team is continuing to refine its methods and collect more data. Their work could help solve one of the biggest mysteries in modern science.

"All of this comes down to something we can't see pulling on the matter we can see," Kopec says. "We know dark matter is out there. We're getting closer to proving it."