  Axion Like Particles (ALPs), postulated to solve the strong-CP problem, are
predicted to couple with photons in the presence of magnetic fields, which may
lead to a significant change in the observed spectra of gamma-ray sources such
as AGNs. Here we simultaneously consider in the same framework both the
photon/axion mixing that takes place in the gamma-ray source and that one
expected to occur in the intergalactic magnetic fields. We show that
photon/axion mixing could explain recent puzzles regarding the observed spectra
of distant gamma-ray sources as well as the recently published lower limit to
the EBL intensity. We finally summarize the different signatures expected and
discuss the best strategy to search for ALPs with the Fermi satellite and
current Cherenkov telescopes like CANGAROO, HESS, MAGIC and VERITAS.
