When searching for CMBR all articles start by stating as a fact that it is a remnant of the Big Bang.
Why? Why is it well understood and accepted? Why sure that it's not an effect of something else yet undiscovered?
For example models in which gravity is caused by quantum entanglement is a hot topic nowadays. Dark matter and dark energy are still kind of a mystery. Why is it sure that there won't ever be a more fundamental explanation of the CMBR other than the Big Bang? I don't know, like microwave glowing is a property of space over large scales, the same way expansion is.
(don't get hung up on the example, I don't want to propose alternatives. I'm interested in the "why", not the "why nots" of alternatives)