Question edited on 27.06.2018 for clarity The Cosmic Microwave Background (CMB) is often called the radiation leftover from big bang, or more precisely, the photons decoupled from the thermal bath when the first atoms were formed.
When we measure the radiations i.e., the flux of photons in a given microwave range (say, 0.1cm to 70cm, for example), in deep sky, there are radiations coming from different galactic sources. When one carefully eliminates photon contributions from these sources and then makes a plot of the intensity versus frequency, one obtains a curve that mimics a blackbody (BB) radiation spectrum. Using that one estimates the temperature to be around $2.7$K.
Back in the time when the universe was younger, this background radiation had a much higher temperature. Then it eventually cooled down with the expansion of the universe to the temperature $2.7$K.
$\bullet$ But apart from being affected by the expansion of the universe, shouldn't the CMB photons be continually contaminated by various luminous sources in the galaxy? Do we have any reason to think that galaxies don't emits photons of microwave range and mix with CMB photons? If yes, they are not really the old photons that were decoupled in the early Universe, and it seems impossible to separate the photons CMB from other microwave radiations that are probably mixing with it.
$\bullet$ Also, what is the reason to think that those old photons have survived till today since decoupling in spite of being (perhaps) continually absorbed by the interstellar medium?
$\bullet$ It is assumed that CMB photons are affected only by expansion. How is it possible that this CMB exists without any interaction with other photons coming from stars etc? Photons can interact with photons at the higher order in QED perturbation theory even though the probability of interaction is highly suppressed.