The axion is a hypothetical elementary particle, arising from our best theoretical solution to the strong CP problem. The axion can be thought of as a very light cousin of the neutral pion, importantly possessing a two-photon coupling. It is believed to be very light, between 1–1000 micro-eV in mass (about 10-42 to 10-39 kg). A very light axion represents an excellent dark matter candidate. Pierre Sikivie’s article “The Pool-Table Analogy for Axion Physics”  presents an intuitive picture for how the axion arises in physics, and how we might be able to find it. A practitioner’s discussion of the theory and experimental avenues to the axion is presented in Ref. 
 P. Sikivie, “The Pool-Table Analogy to Axion Physics“, Physics Today 49 (1996) 22-27.
 P. Graham, I. Irastorza, S. Lamoreaux, A. Lindner, K. A. van Bibber, “Experimental Searches for the Axion and Axion-Like Particles”, Annual Review of Nuclear and Particle Science 65 (2015) 485-514.