If antimatter galaxies existed somewhere in the universe, their light would be indistinguishable from that of matter galaxies. However, along the boundary between matter and antimatter regions, annihilation with the subsequent production of energetic photons (gamma radiation) would be detectable. 30 years of research have placed bounds on how far away such hypothetical antimatter domains of the universe might be, and it is now deemed unlikely that any region within the observable universe is dominated by antimatter.