It is well known that neutrons (outside of atoms) decay with a half-life of around 15 minutes – traditionally it was thought, by beta-decay into a proton, an electron and an antimatter neutrino.
But the two methods of measuring the neutron lifetime have produced different results. One method simply bottles neutrons magnetically, and counts how many are left after certain times. The other method detects protons – the supposed decay product – to infer the lifetime.
A new theory to explain why these two methods give different results, is that neutrons might, less frequently, decay into dark matter particles, possibly emitting a gamma ray in the process.
Experiments are already underway to establish whether there is any gamma ray emission. The significance is that not only would it explain the lifetime measurement discrepancy, but would also give physicists their first real handle on dark matter.