Physics Colloquium 25 November 2016 delivered by Dr Jamie Holder The gamma-ray band of the electromagnetic spectrum probes some of the most extreme environments in the Universe. Photons of these very-high energies can only be produced by the interactions of subatomic particles that have been accelerated to almost the speed of light. This acceleration occurs in a surprisingly wide variety of astrophysical sources: close to black holes and neutron stars, in the blast waves of supernova explosions, and in the relativistic jets of active galaxies. Gamma-ray emission might also result from the interactions of dark matter particles, and so provide a non-gravitational method to detect dark matter in the Universe and to determine its nature. Dr Holder will describe the detection methods for gamma-ray astronomy and highlight some of the most exciting results from the VERITAS observatory, which has been studying astrophysical sources from a mountain in Arizona since 2007. He will also describe the status and prospects for the Cherenkov Telescope Array, a next-generation gamma-ray observatory on a much larger scale.