Google "cosmic microwave background radiation."
One thing that is very clear is that it is unclear.
Even our so called classical physical models are useless when describing nature at the fundamental level.
We don’t have a object. We only have a strange mathematical equation that only describes that object in a general sense. And it gets weirder and stranger the further that we dig into it.
The point here is that we take what we perceive with our senses way too much for granted. And the real world is far from what we think it is even with our imperfect very abstract and elaborate mathematical equations.
Is CMB radiation constant, or decreasing ?