Deep underground, in a former mine, sits a tank of liquid xenon. It catches invisible particles that stream through the Earth every day. Over two and a half years, the tank felt one strong jolt, as if someone had knocked on it from the inside. That hit doesn't look like random noise, and scientists are trying to figure out what caused it.
There are two explanations, like two different keys to one lock. The first key is heavy and breaks itself on impact: a dark matter particle changes its mass, so weak jolts almost never happen. The second key is light but flies at a furious speed, because it was born in the decay of a heavier particle. To tell the keys apart, researchers look at how often weak and strong jolts occur.
So far scientists have just one hit, and that's too little to pick the right key. But if the detector runs longer and catches a few more hits, the picture will clear up. Then the tank in the mine will be able to say who is knocking in the dark.
For boosted dark matter with a pseudoscalar-pseudoscalar interaction, 22% of events fall into the 225-271 keV window, where the recorded candidate lies, whereas for the scalar operator only 0.14% do.
The idea of boosted dark matter resembles plots in which invisible particles gain energy from the decay or annihilation of heavier counterparts, as in some stories about dark sectors, but there is no direct reference to a known work of science fiction.