High-energy astrophysics - Cosmic rays without end

Cosmic rays are high-energy particles, mostly protons and atomic nuclei, that strike the Earth from space. The most energetic of all, with energies above about 1019electron volts, were thought to come from quasars and other distant, active galaxies. But it now appears that their sources are within our Galactic halo — perhaps from the decay of exotic supermassive particles.