  Axion-like particle (ALP) dark matter shows distinctive behavior on scales
where wavelike effects dominate over self-gravity. Ultralight axions are
candidates for fuzzy dark matter (FDM) whose de Broglie wavelength in
virialized halos reaches scales of kiloparsecs. Important features of FDM
scenarios are the formation of solitonic halo cores, suppressed small-scale
perturbations, and enhanced gravitational relaxation. More massive ALPs,
including the QCD axion, behave like CDM on galactic scales but may be clumped
into axion miniclusters if they were produced after inflation. Just as FDM
halos, axion miniclusters may host the formation of coherent bound objects
(axion stars) by Bose-Einstein condensation. This article presents a selection
of topics in this field that are currently under active investigation.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies