On June 16, 2023, 39 seconds after 3:22 P.M. and one mile beneath the Black Hills of South Dakota, there was a sudden flash of light.
It happened in a dark, sealed tank of extremely cold liquid in a cavern where miners once chiseled gold from the walls. Caught by the tank’s many cameras, the flash also sent an electric charge through hair-thin wires submerged in the liquid.
The tank is the centerpiece of the LUX-ZEPLIN (LZ) experiment, one of several ongoing attempts by physicists to snare and study dark matter, a shadowy something they suspect is everywhere—yet somehow seems nowhere to be found. They’ve sought it for the past 40 years by placing detectors in some of Earth’s most silent and secluded spaces, places where dark matter’s whispers might register. But every time so far, they’ve come up empty, with every burst of light or surge of electricity having a more ordinary explanation.
This flash from three years ago is different. The charge and light together seem to indicate a violent collision between an unknown particle and the nucleus of one of the detector’s atoms. The event bears almost no resemblance to any of the “backgrounds”—mundane sources of flashes that mimic dark matter—which the LZ collaboration’s hundreds of scientists have spent their careers battling.
Yet the collision is also out of sync with the kind of dark matter signal they’ve been searching for. That is, it’s a total mystery. But after four decades of nothing, a new mystery is welcome.
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