Diffuse extragalactic background radiation

The diffuse extragalactic background radiation (DEBRA) refers to the photon field of extragalactic origin that fills our Universe. It contains photons whose energies span more than twenty orders of magnitude, from 10−7 eV to more than 100 GeV. This range covers everything from the microwaves emitted by free hydrogen atoms to ultra high-energy gamma rays, which can only be emitted by the most powerful physical processes in the modern universe such as kilonovas and merging black holes.

Source: Wikipedia — Diffuse extragalactic background radiation (CC BY-SA 4.0)