The origin of cosmic rays and the Vela gamma-ray excess

The recent observation of excess γ-ray emission from the region around the Vela supernova remnant provides the most direct evidence so that supernovae produce sufficient energy in relativistic particles to be the source of cosmic rays. We show that an estimate of the cosmic ray energy around Vela suggests that either only a fraction of galactic supernovae is the source of most cosmic rays, or only a fraction of the matter traversed by cosmic rays is in the interstellar medium