The extragalactic background light and the gamma-ray opacity of the universe

► We review the current limits and detections of the extragalactic background light (EBL). ► We present a list of gamma-ray sources and their GeV-TeV spectral characteristics. ► Determination of the EBL spectrum from gamma-ray sources requires knowledge of the intrinsic source spectra, and vice versa. ► The universe is transparent to ⩽400 GeV gamma-rays up to redshifts of 0.4, and to ⩽2 TeV gamma-rays to redshifts of ∼0.2. ► The EBL spectrum can be used to set constraints on cosmological energy releases and gamma-ray production mechanisms.