Axion-like particles have good theoretical motivation and are characterised by conversion to photons in astrophysical magnetic fields.
Galaxy clusters are the most efficient convertors of axion-like particles to photons in the universe.
I discuss the physics and phenomenology of ALPs, and describe their astrophysical implications, with particular reference to the
recently observed 3.5 keV X-ray line that is a candidate for a dark matter decay line. I discuss interpretations of this line in terms of
dark matter decaying to an axion-like particle, that then converts to a photon in cluster magnetic fields, and describe the compatibility of this scenario
with data and the different phenomenology for cool-core and non-cool-core clusters.