  The imprint of cosmic backgrounds in the gamma ray spectra of blazars has
recently been detected by H.E.S.S. and Fermi-LAT, opening the way to studies of
gamma-ray propagation on cosmological scales. This proceeding discusses the
current constraints on the extragalactic background light (EBL), the effort to
increase the collection of blazars detected at TeV energies, and a crucial part
of the science case of next-generation instruments: gamma-ray cosmology.
