Axions and similar axion-like particles are among the theoretically most well-motivated extensions of the Standard Model of particle physics, and may also comprise dark matter. Unfortunately, they are also very difficult to detect experimentally. Extreme astrophysical environments such as supernovae, neutron stars, or vast, magnetised galaxy clusters may carry subtle signals of axions. Detecting such signals, or conclusively ruling them out, will constitute a significant advance in our understanding of fundamental physics and cosmology. This project aims to determine the theoretical predictions for signals from axions and ALPs in cosmological or astrophysical environments, and then search for these in observational data.