Cosmic rays this powerful were thought to be impossible. The reason for this is the cosmic microwave background, or CMB. For practical purposes, it makes sense to think about outer space as being essentially empty. But in the sneaky and strange world of the subatomic, space is not empty. Instead, it’s filled with low-energy photons. These photons are the CMB, or the dim glow that still persists from the cataclysm that was the big bang. Think of the CMB like the early morning embers of last night’s bonfire; it’s not a raging inferno, but it’s still giving off just a little heat.
The universe contains some pretty powerful phenomena — things like quasars and gamma-ray bursts. Powerful cosmic phenomena frequently have powerful magnetic fields, which can accelerate particles to ridiculous speeds. The problem is that these accelerated particles can’t get very far — in astronomical terms, anyway — before being slowed down by the CMB to speeds that are admittedly still incredibly fast, but not OMG-fast.
This leaves scientists with a dilemma. Based on the measurements of the Oh-My-God particle’s speed, it had to have originated from relatively nearby (again, in astronomical terms). But the only natural events and objects powerful enough to create the Oh-My-God particle are so big and so energetic that we would already know if we had one in our cosmological backyard, and we haven’t found anything yet that fits the bill. So then where in the world did this thing come from?