For half a century, our view of the world has been based on the standard model of particle physics. However, this view has been challenged by theories1 that can overcome some of the limitations of the standard model. These theories allow neutrinos to be Majorana particles (that is, they are indistinguishable from their own antiparticles) and predict the existence of weakly interacting massive particles (WIMPs) as the constituents of invisible ‘dark matter’ in the Universe. Majorana neutrinos mediate a type of nuclear decay called neutrinoless double-? decay, an example of which is neutrinoless double electron capture. A crucial step towards observing this decay is to detect its standard-model equivalent: two-neutrino double electron capture.