Evidence for an axion-like particle from blazar spectra?
  Observations with the Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III and VERITAS have shown that the Universe is more transparent than expected to gamma rays above 100GeV. As a natural explanation, the DARMA scenario has previously been proposed, wherein photons can oscillate into a new very light axionlike particle and vice-versa in the presence of cosmic magnetic fields. Here we demonstrate that the most recent observations further support the DARMA scenario, thereby making the existence of a very light axion-like particle more likely. 
Marco Roncadelli, Alessandro De Angelis, Giorgio Galanti, Massimo   Persic
astro-ph.HE astro-ph.CO hep-ph