  Studying the propagation of gamma rays on cosmological distances encompasses
a variety of scientific fields, focusing on diffuse radiation fields such as
the extragalactic background light, on the probe of the magnetism of the
Universe on large scales, and on physics beyond the standard models of
cosmology and particle physics. The measurements, constraints and hints from
observations of gamma-ray blazars by airborne and ground-based instruments are
briefly reviewed. These observations point to gamma-ray cosmology as one of the
major science cases of the Cherenkov Telescope Array, CTA.
