The transparency of the universe for very high energy gamma-rays
  The propagation of very high energy gamma-rays ($E>100$~GeV) over
cosmological distances is suppressed by pair-production processes with the
ubiquitous extra-galactic soft photon background, mainly in the optical to near
infra-red. The detailed spectroscopy of gamma-ray emitting blazars has revealed
the signature of this absorption process leading to a meaningful measurement of
the background photon field which is linked to the star-forming history of the
universe. Deviations from the expected absorption have been claimed in the
past. Here the status of the observations is summarized, an update on the
search for the persisting anomalous transparency is given and discussed.
