Jaeyoung Cho

No one has ever seen dark matter, but it should pass through detectors like wind through leaves. Sometimes, very rarely, a dark matter particle bumps into an atomic nucleus, and then the detector must catch that jolt. The trouble is that light dark matter particles can hardly move heavy nuclei: it's like trying to knock down a bowling pin by throwing a feather at it.

Deep inside a mountain, where cosmic-ray muons are muffled by a thick layer of rock, stands a detector that catches not light but its absence. Positronium is a tiny atom in which an electron and its antiparticle, the positron, circle each other like a pair of dancers. Usually they come together and vanish in a flash of two or three gamma quanta.