In nuclear transitions of beryllium-8, helium-4, and carbon-12, physicists from the ATOMKI collaboration have been seeing extra events since 2020—as if, when an excited nucleus decays, something else is sometimes born besides the usual photons. The energy of this "something" is always the same: about 17 million electron-volts. In 2025, a similar excess was spotted by the PADME experiment near Rome, but in collisions of positrons with electrons. Two different detectors, two different processes, yet the extra events appear at the same energy.

The simplest explanation is a new particle, dubbed X17. It would have to be very light, neutral, and short-lived: born inside the nucleus, it immediately decays into an electron and a positron. If X17 is real, it doesn't fit into the Standard Model — our best guide to elementary particles. We'd have to extend it.

The author of the work asks: what if X17 isn't just an exotic curiosity, but a bridge to the hidden sector? That's what physicists call a world of particles that feel neither the strong nor the weak nor the electromagnetic force — only gravity. It's from such particles that dark matter might be made. If X17 can talk to both ordinary particles and the denizens of the hidden sector, it could explain why dark matter is so abundant, yet we can't see it directly.

So far, no experiment has conclusively confirmed X17: other facilities, like MEG II, see no anomalies. But the coincidence of ATOMKI and PADME results makes us look more carefully. We might be on the brink of a discovery that connects two of physics' greatest mysteries — the nature of dark matter and the workings of the world beyond the Standard Model.