  Very high energy (VHE, energy >~ 100 GeV) {\gamma}-rays originating from
extragalactic sources interact with low energy photons of background radiation
fields and produce electron-positron pairs. Alternatively, in the presence of
ambient magnetic fields, they can convert into hypothetical axion-like
particles (ALPs), pseudo-scalar spin-0 bosons, predicted by extensions of the
standard model. These particles propagate unimpeded over cosmological
distances. Here, the effect of photon-ALP oscillations in magnetic fields of
galaxy clusters and the Milky Way on VHE {\gamma}-ray spectra is studied. It is
shown that this mechanism can lead to a substantial enhancement of the VHE flux
and a spectral hardening, thus effectively reducing the opacity of the Universe
to VHE {\gamma}-rays.
