So there’s “Cosmic Radiation” and “Cosmic Background Radiation” – The CBR, the background radiation is leftover from the big bang – it’s [source is everywhere](https://profoundphysics.com/did-the-big-bang-happen-everywhere-at-once/) – we detect it absolutely everywhere we look, in every direction.
The big bang happened when the universe was essentially one point, and the expansion happened everywhere, and is still happening everywhere. It’s basically just left-over energy from billions of years ago. There isn’t “A source” it’s just kinda, there, in the background, all the time.
Then there’s other sources of “Cosmic Radiation” – massive scary space stuff like nearby stars, supernovae, quasars, black holes.
Obviously this type of radiation is localized and directional, when we get hit with a solar flare from the sun, it’s a pretty obvious bit of Cosmic Radiation.
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