The particles, known as cosmic rays, can show up with energies a million times higher than the biggest particle accelerators on Earth can produce. Astrophysicists believed that only two sources could make them: supermassive black holes in active galaxies, or so-called gamma ray bursts. A study in Nature has now all but ruled out gamma ray bursts as the cause.And this from the Reddit thread:
If the SMBH at the center of the galaxy is the source, than we should observe the highest energy cosmic rays coming from that direction. We don't. They come roughly equally from every direction... Here's the really crazy part. Outside of a gamma-ray burst or a supermassive black hole, we know of nothing that can produce protons at that speed. Yet, they seem to be coming from all around us. Because of interactions with background radiation, there is an upper limit to how far away such a high energy proton could come from: and it is within our own galaxy.