The dark matter particle can be a QCD axion or axion-like particle. A locally over-densed distribution of axions can condense into a bound Bose–Einstein condensate called an axion star, which can be bound by self-gravity or bound by self-interactions. It is possible that a significant fraction of the dark matter axion is in the form of axion stars. This would make some efforts searching for the axion as the dark matter particle more challenging, but at the same time it would also open up new possibilities. Some of the properties of axion stars, including their emission rates and their interactions with other astrophysical objects, are not yet completely understood.