As I understand it there can be said to be two "kinds" of electromagnetic radiation filling up the universe. First you have the primordial radiation from a time soon after Big Bang. Then you have "newer" radiation from the fact that you start up with very diffuse clouds of mainly hydrogen that falls together and forms planets, stars, black holes etc. basically converting potential energy to heat radiation and in the stars you of course also have fusion producing heat that radiates away.
Can you say something like:
X percent of the energy of the electromagnetic radiation in the universe is primordial and (100 -x) percent of the energy of the electromagnetic radiation in the universe is of newer origin, and if so, what is X?
I mean if you "magically removed" all "primordial radiation" some 13 billion years ago we would still have a lot of radiation in the universe today.