For every atomic particle there exists a complementary particle with equal mass but opposite charge: such is the case, for instance, with electrons and positrons, protons and antiprotons, neutrons and antineutrons. For each pair of particles, one is designated as ordinary matter and the other as antimatter (the one exception being Majorana fermions, chargeless particles – such as photons – that act as their own antiparticles).

Astrophysics tells us that the Big Bang should have produced equal amounts of matter and antimatter, but this is clearly not the case. The reason for this imbalance is a still a mystery, but may lie in the nature of the neutrino, a nearly massless subatomic particle that – just like the photon – may act as its own antiparticle. If neutrinos are indeed Majorana fermions, they may have decayed asymmetrically in the early universe and given rise to the preponderance of matter over antimatter that we see today.