Picture yourself tuning a radio in the dead of night, hoping to catch the whisper of some distant station. Instead, you get nothing but the hiss of static. That is roughly how physicists listened for a signal from dark photons: 36 hours of silence, and that silence turned out to be worth more than any noise.
Dark photons are particles that scientists think could make up dark matter. Dark matter is the invisible stuff that holds galaxies together but gives off no light. Ordinary photons are particles of light. Dark photons mix with ordinary ones only very faintly. To spot that, you need an extremely sensitive detector. The DALI prototype is a stack of 20 plates, something like a set of shelves. It works as two mirrors with a wave bouncing back and forth between them, building up strength.
Until now, nobody knew how strongly dark photons could mix with ordinary light in this range. Now a limit is in place: the mixing must be weaker than 6.8×10⁻¹⁴. That number is tiny, and it tells us that dark photons, if they exist, interact with our world even more faintly than we could detect. For the future large detector, this is good news: it will be able to search for even weaker signals.
A sensitivity of χ ≲ 6.8×10⁻¹⁴ means that if a dark photon and an ordinary photon mixed with that strength, there would be roughly one dark photon for every trillion ordinary photons. It is like hunting for one particular grain of sand on a beach hundreds of kilometers long.
The idea of hidden photons that permeate our Universe and only rarely reveal themselves echoes the concept of a "dark sector" in science fiction, for instance in Liu Cixin's novel The Dark Forest, where invisible particles play a key role in the plot.