Gamma-ray observations of Seyfert galaxies and the problem of the extragactic gamma-ray background

Observations of the gamma-ray (GR) emission from Seyfert galaxies and the origin of the diffuse cosmic GR component are critically reviewed. A comparison of observations of NGC 4151 at X-ray energies and at GR energies is presented. In addition, synchrotron self-Compton models are discussed as a possible explanation for the cutoff in the GR spectrum. The origin of the diffuse cosmic GR component is investigated as the possible result of various processes, including quantum-mechanical decay of small black holes and matter-antimatter annihilation in a baryon symmetric universe. Also discussed is continuum GR emission from our own galaxy, from the quasar 3C273 and from the Seyfert galaxy NGC 4151