Modern science, especially the Big Bang Cosmology, tends to explain little (observable) facts on the basis of ‘already known’ realities of whole universe. For example, our scientist (Georges Lemaître) already knows that ‘Universe is Expanding’. On the basis of this ‘already known’ larger fact of reality, he ‘explains’ observed redshifts of galaxies (little fact).
Title of his (1927) paper is “A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-galactic Nebulæ”.
Thus before explaining redshifts, he is already aware of ‘larger fact’ that radius of universe is increasing. On the basis of already known ‘larger fact’, he is explaining or interpreting a little (observable) fact.
CMBR is also explained in this way. Science ‘already knows’ that universe originated from a point that, after 380000 years, had become 43 million light years in diameter. Science ‘already knows’ that this universe first time emitted light (photons). Science ‘already knows’ that that universe had such and such temperature. Science ‘already knows’ that universe was expanding and that light was also expanding. Science ‘already knows’ that now the original light must have this much temperature.
On the basis of these ‘already known’ larger realities of universe, CMBR (a little observable fact) is explained.
‘Predictions’ of Big Bang Cosmology did not even match with observed fact. Therefore, adjustments of dark matter/dark energy are applied to get matching results. This is the actual prevailing scientific methodology whose examples are quite apparent in different aspects of the Big Bang Cosmology. And they say that it is science.