The existence of a dark, cold (i.e., nonrelativistic), and nearly
collisionless matter component called "dark matter" is a meanwhile well
established part of the standard model of cosmology, confirmed by
independent probes ranging in scale from the Hubble length to galactic
radii and in time from the first seconds to the last million years of
cosmic evolution. Yet all attempts to identify the nature of dark
matter, for instance in direct or indirect detection experiments or by
direct production at the LHC, have so far been unsuccessful.
Axions are among the best motivated candidates for dark matter particles. They are a natural by-product of the Peccei-Quinn solution to the strong CP problem of quantum chromodynamics (QCD).
If the Peccei-Quinn symmetry is broken after inflation, large axion isocurvature perturbations can collapse very early and form small, gravitationally bound structures called axion miniclusters. Predicting the their expected mass function and density profiles is important for astronomical and experimental axion searches.