  Dark matter constitutes about $23\%$ of the total energy density of the
universe but its properties are still little known besides that it should be
composed by cold and weakly interacting particles. Many beyond standard model
theories can provide proper candidates to serve as dark matter and the axion
introduced to solve the strong CP problem turns out to be an attractive one. In
this paper, we briefly review several important features of the axion and the
axion dark matter.
