How can we use CMBR as evidence of the Big Bang Theory?
The CMBR proves that the universe was once filled with high energy photons.
The early universe would have been full of high energy gamma rays. They would have been everywhere travelling in all directions;
As time passed these gamma rays would have lost energy to become microwaves today.
As there are microwaves of the same energy coming from every direction this proves that the universe was once full of high energy photons. This is seen as proof of the big bag.