Diffuse celestial radiation extending up to energies of at least 200 MeV. On a coarse scale, the gamma-ray background appears to be uniformly distributed (isotropic), indicating that it originates outside our Galaxy. Its low intensity rules out the steady-state theory of the Universe. Three possible sources of at least part of the radiation are the annihilation of matter and antimatter, active galaxies, and the explosion of primordial black holes.