Axions are theoretical elementary particles first proposed by Peccei-Quinn theory in 1977 to resolve Charge-Parity Violation in Quantum Chromodynamics. They were named after a brand of detergent, because the problems with QCD were supposed to have been 'cleaned up'. Several experimental searches have been done for axions, with at least one claiming positive results. Their existence is currently unestablished.
Axions are supposed to have extremely low mass, no electrical charge, and low interaction cross-sections for the strong and weak interactions. They are supposed to transform to and from photons in the presence of strong magnetic fields, which has formed the basis of detection experiments.
If they exist, axions are supposed to have been created in enormous quantities during the big bang, and thus primordial axions provide one possible explanation for dark matter.Sources: