Axion Dark Matter

The physical nature of the cosmic dark matter remains perhaps the most vexing mystery of contemporary cosmology. One well-motivated particle-physics solution is provided by the hypothesis of axions, very weakly interacting and very low-mass particles, that would simultaneously explain why quantum-chromodynamics perfectly
respects the symmetry between matter and anti-matter ("strong CP problem"). The theoretical motivation for axions, their cosmological role, experimental searches, and astrophysical limits will be explained and reviewed.