A neutrino is an elementary particle that is able to pass through ordinary matter, but is difficult to detect. Physicists have hypothesised that there is a particular type of neutrino – a sterile neutrino – that cannot be detected at all by their instruments. Relevant evidence comes from supernovae, i.e. exploding stars. If sterile neutrinos did exist, supernovae would shoot them out, and the recoil from this blast would send pulsars (rotating stars that emit a beam of electromagnetic radiation) travelling at high speed through the universe. It turns out that astronomers observe precisely that: pulsars whizzing through the universe at speeds of thousands of kilometres per second.