In deep cold, at a temperature just two degrees above absolute zero, a copper cavity the size of a saucepan listens to the silence of the universe. This cavity is the heart of the Supax detector prototype, built to search for axions: particles that no one has ever seen but that could make up the invisible dark matter holding galaxies together.

Axions are like ghosts: they barely interact with ordinary matter. But if you place the cavity in a strong magnetic field, an axion can transform into a photon — a particle of light. Then a faint radio signal would arise in the cavity, like a whisper in a quiet room.

The scientists cooled the cavity to 2 kelvin to eliminate thermal noise and switched on a 12-tesla magnet. They searched for a signal at a frequency corresponding to an axion mass of about 34 microelectronvolts: that's billions of times lighter than an electron. But no whisper was heard. However, it is now known that if the axion exists, it interacts with photons even more weakly than previously thought: the upper limit has been lowered to 1.6×10⁻¹³ GeV⁻¹. The same instrument also ruled out dark photons, another dark matter candidate, with a mixing parameter greater than 1.4×10⁻¹².

Silence is also a result. It narrows the search, like combing a forest: if you don't find anything in one grove, you move to the next. The Supax prototype showed that the technology works, and now the team is preparing a full-scale detector for a wider mass range. Perhaps it will be the one to hear the first whisper of dark matter.