Some choice interesting quotes about the potential origin of such particles:

> Instead, theorists generally expect that the most energetic cosmic rays rev up over millions of years in unidentified accelerators the size of galaxies.

> Physicists think that the highest energy cosmic rays cannot come from more than 500 million light-years away, as interactions with lingering radiation from the big bang ought to snuff out cosmic rays from more distant sources. But no obvious candidate for a nearby cosmic accelerator lies directly in line with the hotspot, Sokolsky says. He notes, however, that in that region a filament of galaxies kinks toward Earth and speculates that magnetic fields in that string might help rev up particles.