The propagation of very high energy gamma-rays (E > 100 GeV) over cosmological distances is limited by pair-production processes with the ubiquitous extra-galactic soft photon background 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 anomalous transparency is given and discussed.
The propagation of very high energy gamma-rays (E > 100 GeV) over cosmological distances is limited by pair-production processes with the ubiquitous extra-galactic soft photon background 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 anomalous transparency is given and discussed.