Axion field at the big bang, illustration. Axions are hypothetical elementary particles suggested to explain why charge-parity (CP) symmetry is preserved in quantum chromodynamics (QCD). They have since been proposed as a leading candidate for dark matter and the reason why there is more matter than antimatter in the universe. The axion particle (white) has settled at the lowest energy (stable) state after rotating (multicoloured lines) around its field (grid). Interactions as it rotated would have given rise to particles. As the rotation was in one direction it would have produced matter particles rather than antimatter particles, accounting for matter-antimatter asymmetry. Before coming to rest the axion would have oscillated around zero, generating axion dark matter. Axions have masses billions of times less than that of an electron.