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We reconstruct \gamma-ray opacity of the Universe out to z<3-4 using anextensive library of 342 observed galaxy luminosity function surveys extendingto high redshifts. We cover the whole range from UV to mid-IR (0.15-25mic)providing for the first time a robust empirical calculation of the\gamma\gamma-optical depth out to several TeV. Here, we use the same databaseas Helgason et al. 2012 where the EBL was reconstructed from luminosityfunctions out to 4.5mic and was shown to recover observed galaxy counts to highaccuracy. We extend our earlier library of LFs to 25mic such that it covers theenergy range of pair production with \gamma-rays 1) in the entire Fermi/LATenergy range, and 2) at higher TeV energies probed by ground-based Cherenkovtelescopes. In the absence of significant contributions to the cosmic diffusebackground from unknown populations, such as the putative Population III erasources, the Universe appears to be largely transparent to \gamma-rays at allFermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already atz<0.2 and reaching \tau~10 at z=1. Comparing with the currently availableFermi/LAT GRB and blazar data shows that there is room for significantemissions originating in the first stars era.

Reconstructing the \gamma-ray Photon Optical Depth of the Universe to z~4 from Multiwavelength Galaxy Survey Data