  The axion has emerged in recent years as a leading particle candidate to
provide the mysterious dark matter in the cosmos, as we review here for a
general scientific audience. We describe first the historical roots of the
axion in the Standard Model of particle physics and the problem of
charge-parity invariance of the strong nuclear force. We then discuss how the
axion emerges as a dark matter candidate, and how it is produced in the early
Universe. The symmetry properties of the axion dictate the form of its
interactions with ordinary matter. Astrophysical considerations restrict the
particle mass and interaction strengths to a limited range, which facilitates
the planning of experiments to detect the axion. A companion review discusses
the exciting prospect that the axion could {indeed} be detected in the near
term in the laboratory.
