Why is Cosmic Microwave Background Radiation attributed to the Big Bang? 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)
Thank you
 A: The reason that relates CMB to inflation and Big Bang is that this microwave background is nearly homogeneous even though it covers regions which would be causally disconnected if the universe did not undergo a period of exponential expansion. The radiation coming from zones very distant to each others is the same: how could it be possible if their distance is greater than what light could have possibly travelled in the age of the universe? 
By theorizing the inflation one roughly says that enough time has passed for those regions to actually be in causal contact before they emitted the CMB: in this way it is possible that in every part of the universe the cosmic microwave background has been emitted with the same "temperature". Intuitively if you have two tanks of water than were never in contact how could you expect them to be at thermal equilibrium? They must have been in contact with each other in the past.
Of course there are alternative ideas to inflation to solve this problem (for instance non constant speed of light), but inflation also explains anisotropies in the CMB so is really strongly motivated. If you want to know more have a tour on Wikipedia looking for inflation or "horizon problem".
