When an electron and a positron meet, they circle each other like a pair of dancers. This tiny atom is called positronium. Usually the dancers come together and vanish in a bright flash of light: a gamma-ray photon. But sometimes, scientists suspect, instead of a flash an invisible particle is born, carrying the energy away unnoticed. The KAPAE II detector was built to catch exactly this case, when there is no flash.
To notice the absence of a flash, you first have to remove all the stray flashes. So the detector was lowered into an underground laboratory at a depth of 1000 meters, where cosmic rays barely reach. And the crystals that catch the light were cooled to −37.5 °C: when cold, they glow brighter and more precisely. As a result, interference dropped by a factor of 3000 compared with the surface. Now, if positronium disappears without light, the detector will notice, and the silence will become a loud discovery.
Positronium is the lightest known atom: its mass is 2000 times smaller than that of a hydrogen atom. The lifetime of ortho-positronium in vacuum is about 142 nanoseconds, but in the detector it annihilates faster because of interactions with matter.
The idea of invisible particles carrying away energy echoes the "dark matter" of science fiction, for example in Liu Cixin's novel The Three-Body Problem, where an invisible substance affects physical processes. In a real experiment, however, the goal is a rigorous search for specific candidates, not fictional substances.