  Gamma rays from TeV blazars have been detected by ground-based experiments
for more than two decades. We have collected the most extensive set of archival
spectra from these sources in order to constrain the processes affecting
gamma-ray propagation on cosmological distances. We discuss our results on the
diffuse photon field that populates universe, called the extragalactic
background light, on the expansion rate of the Universe, and on fundamental
physics in the form of axion-like particles and Lorentz-invariance violation.
Specifically, we present a spectrum of the extragalactic background light from
0.26 to 105 microns constructed from the gamma-ray observations, we measure a
value of the Hubble constant compatible with other estimates, and we constrain
the energy scale at which Lorentz-invariance violation impacts gamma-ray
absorption by the extragalactic background light to be larger than sixty
percent of the Planck scale.
