Axions, conceptual illustration. The axion is represented here as a detergent capable of cleaning up dark matter related problems and the strong CP-symmetry problem. 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. Axions have masses billions of times less than that of an electron.