  Several extensions of the Standard Model predict the existence of Axion-Like
Particles (ALPs), very light spin-zero bosons with a two-photon coupling. ALPs
can give rise to observable effects in very-high-energy astrophysics. Above
roughly 100 GeV the horizon of the observable Universe progressively shrinks as
the energy increases, due to scattering of beam photons off background photons
in the optical and infrared bands, which produces e+e- pairs. In the presence
of large-scale magnetic fields photons emitted by a blazar can oscillate into
ALPs on the way to us and back into photons before reaching the Earth. Since
ALPs do not interact with background photons, the effective mean free path of
beam photons increases, enhancing the photon survival probability. While the
absorption probability increases with energy, photon-ALP oscillations are
energy-independent, and so the survival probability increases with energy
compared to standard expectations. We have performed a systematic analysis of
this effect, interpreting the present data on very-high-energy photons from
blazars. Our predictions can be tested with presently operating Cherenkov
Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO III as well as with
detectors like ARGO-YBJ and MILAGRO and with the planned Cherenkov Telescope
Array and the HAWC-ray observatory. ALPs with the right properties to produce
the above effects can possibly be discovered by the GammeV experiment at
FERMILAB and surely by the planned photon regeneration experiment ALPS at DESY.
