The world’s most sensitive dark matter detector has seen a single unexplained signal, scientists reported on September 1 at the TeVPA conference in Japan.

The LUX-ZEPLIN experiment, buried 1.6 kilometres underground in South Dakota, uses 10 tons of liquid xenon to catch the faint flashes dark matter particles might produce.

After 220 days of live observations, the detector recorded one event at 248 keV with a significance of 2.6 sigma, said researcher Sam Eriksen of the University of Bristol. That is well below the threshold for a discovery.

The team says the event is most likely background noise, but they cannot yet explain it. More data will show whether it repeats.

Dark matter makes up most of the universe’s mass but has never been directly detected. LUX-ZEPLIN will keep running to push the search deeper.

Dark matter is thought to outweigh visible matter about five to one, yet decades of searches have found nothing. Each null result narrows the hiding places where the mysterious particles could be.