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The axion came into play in the late seventies in an attempt to solve the strong CP problem, i.e. the absence of CP violation in the strong interaction. It is a pseudoscalar boson whose mass and couplings to standard model particles are inversely proportional to fA, an energy scale related to the spontaneous breaking of a newly introduced simmetry (the so called Peccei and Quinn simmetry).
Several axion models have been proposed, with the common feature of low mass and very weak interactions, making this particle an excellent Dark Matter candidate.