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Introduction to extragalactic sources of very high-energy photons

The launch of the Fermi gamma-ray space telescope and the imaging air Cerenkov telescopes H.E.S.S., MAGIC, and VERITAS have substantially transformed our knowledge of gamma-ray sources in the last decade. The extragalactic gamma-ray sky is teeming with blazars, which are active galactic nuclei whose jet is directed at us. Additionally, there are radio galaxies, starburst and spiral galaxies, and gamma-ray bursts, albeit with smaller numbers. Galaxy clusters have not yet been observed in gamma rays. Here, I will introduce the different gamma-ray emission processes and review what they may tell us about these objects and the underlying acceleration mechanisms. Beyond the study of these fascinating objects, TeV gamma rays from blazars probe the integrated star formation history of the universe. Studies of TeV blazar spectra may provide us with insights into intergalactic magnetic fields or alternatively, may lead us to infer the existence of a novel mechanism that heats the intergalactic medium at late times (for redshifts z<3) and impacts the Lyman-alpha forest and late-time structure formation. The TeV gamma-ray emission may also allow us to probe fundamental physics such as the structure of space time.