And we detect this "cosmic background radiation" coming more or less evenly from all directions, forming a background beyond the stars.

These quasar jets are formed when electrons emitted from a black hole impact with cosmic background radiation left by the big bang, giving astronomers clues about the conditions in the early universe.

UHECRs tend to collide with photons of the cosmic microwave background—pervasive radiation that is a relic of the early universe.