A pot on the stove rings when the water boils. An empty copper cup can also "ring"—at radio frequencies — if a dark matter particle is born inside it. These particles are called axions, and according to one hypothesis, they fill the entire galaxy as an invisible cloud. Inside a metal cavity, an axion occasionally turns into a photon — a particle of light. If the cavity is tuned to the right frequency, the photon resonates like a voice in a bathroom, and a sensitive receiver can pick it up.

The trouble is that the heavier the axion, the higher the frequency and the smaller the cavity must be. For a frequency around 23 gigahertz (a thousand times higher than a microwave oven), you need a cavity the size of a coin. One such coin catches too few particles, and the instrument noise grows at high frequencies. Physicists at the Pacific Northwest National Laboratory (PNNL) decided to assemble a whole battery from these coins — several dozen identical cavities working as one large antenna. Each cavity is like a musician in an orchestra: alone the sound is weak, but together they play loudly and in harmony.

To keep the orchestra from playing off-key, each cavity must be tuned to the same note. A movable rod is inserted inside, like a piston in an organ pipe. Moving it changes the resonance frequency. In the new work, the rod is combined with the antenna that picks up the signal. The result is the world's first battery of tuned cavities operating in the 22.88–22.93 gigahertz range. This corresponds to an axion mass of about 95 microelectronvolts — a hundred billion times lighter than an electron.

So far, the battery hasn't found dark matter, but it has proven that the technology works. The next step is to assemble hundreds and thousands of cavities to hear even the faintest whisper of axions. If they exist, such an orchestra might one day play the long-awaited note.