I found this information most interesting:
VERITAS could not detect M82's cosmic rays directly because they are trapped within the Cigar Galaxy.
Instead, VERITAS looked for clues to the presence of cosmic rays: gamma rays. Gamma rays are the most energetic form of light, far more powerful than
ultraviolet light or even X-rays.
When cosmic rays interact with interstellar gas and radiation within M82, they produce gamma rays, which can then escape their home galaxy and reach
Earthbound detectors...
therefore the VERITAS space telescope does not measure intergalactic
cosmic rays(particles)...it only can measure local 'Milky Way' cosmic radiation... which is 500X less than the target M82 galaxy.
Am i thinking correctly? then what is the 'background' cosmic radiation left over from the big-bang event...is that stuff cosmic protons that
actually enter the galactic membrane ~or~ are we again measuring the gamma rays byproduct ...