Current cosmology is based on CMBR and the idea it is remnant from the big bang, I guess we assume it's the same radiation in the whole universe. My question is how do we know the CMBR is not something local to our galaxy lets say? How do we know in another galaxy the CMBR won't be different? We don't have the possibility to take measurements outside the solar system, not to mention the galaxy. So how do we know the CMBR is uniform and identical throughout the whole universe? It's a bit like being in the middle of the pacific ocean, it will be just water and the horizon everywhere you look. Of course that doesn't mean the whole planet is like that, only that region, that happens to be everything within our range. How do we know it's not the same with the CMBR? Recently I've read that there are other possible sources of the CMBR like giant supernovae scattered radiation or super compressed matter that falls into the event horizon of black holes, that pretty much resembles conditions, similar to those in the beginning of the universe according to the bb theory.