  In the recent years, the number of detected very high energy (VHE: E > 100
GeV) gamma-ray sources has increased rapidly. The sources have been observed at
redshifts up to z = 0.536 without strong indications for the presence of
absorption features in the energy spectra. Absorption is however expected due
to pair-production processes of the propagating photons with the photon bath in
intergalactic space. Even though this photon density is not well known, lower
limits can be firmly set by the resolved emission from galaxy counts. Using
this guaranteed background light, we investigate the behaviour of the energy
spectra in the transition region from the optically thin to the optically thick
regime. Among the sample of 50 energy spectra, 7 spectra cover the the range
from optical depth $\tau < 1$ to $\tau > 2$. For these sources, the transition
to $\tau > 2$ takes place at widely different energies ranging from 0.4 TeV to
21 TeV. Consistently, in all of these sources, an upturn of the
absorption-corrected spectrum is visible at this transition with a combined
significance of 4.2 standard deviations. Given the broad range of energies and
redshifts covered by the sample, source-intrinsic features are unlikely to
explain the observed effect. Systematic effects related to observations have
been investigated and found to be not sufficient to account for the observed
effect. The pair-production process seems to be suppressed in a similar way as
expected in the extension of the standard model by a light (<neV) pseudoscalar
(axion-like) particle.
