  The axion arises in well-motivated extensions of the Standard Model of
particle physics and is regarded as an alternative to the weakly interacting
massive particle paradigm to explain the nature of dark matter. In this
contribution, we review theoretical aspects of dark matter axions, particularly
focusing on recent developments in the estimation of their relic abundance. A
closer look at their non-thermal production mechanisms in the early universe
reveals the possibility of explaining the observed dark matter abundance in
various mass ranges. The mass ranges predicted in various cosmological
scenarios are briefly summarized.
