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. But sometimes, theorists suspect, instead of a flash something is born that carries energy away without a single trace. The KAPAE II detector was built to hear that silence.

The heart of the detector is an array of 25 BGO crystals (bismuth germanium oxide), each the size of a matchbox but 15 centimeters long. When a gamma quantum hits a crystal, the crystal flashes with blue light. The light is captured by silicon photomultipliers, devices that turn a faint flash into an electrical signal. To catch the positron before it annihilates, a thin PEN film as thick as a human hair is used; it acts like a starting pistol, giving the crystals their cue: "Watch closely."

But any detector on the Earth's surface drowns in background noise: cosmic muons pour down like rain, causing false coincidences. That is why KAPAE II was lowered into the underground Yemi laboratory to a depth of 1000 meters. There the muon flux is weakened by more than a factor of 330,000, which is like moving from a noisy street into a soundproof room. In addition, the detector was cooled to −37.5 °C: at that temperature the BGO crystals glow more brightly and for longer, allowing the flash energy to be measured more precisely.

The result exceeded expectations. The background coincidence rate fell from one event per second at the surface to less than one event per hour underground. That is a suppression by more than three orders of magnitude, as if the noise of a crowd turned into the whisper of a single person. After correcting for light leakage between neighboring crystals, the measured spectrum agrees well with computer simulations, confirming that the detector is ready for the main search.

Now KAPAE II will collect data for months, waiting for a rarest of events: positronium decays and no energy appears in the crystals. Such "silence" could mean the birth of a dark-sector particle, for example a dark photon or an axion-like particle. If no signal is found, the limit on the probability of an invisible decay will improve to 10⁻⁸, which is an order of magnitude stricter than before. The trap for the invisible is ready, and the silence inside it will be heard, even if it remains silence.