This is also known as the "3 Degree Background". These photons come from the "Epoch of Recombination" which is observed at very high redshift. These microwave photons were produced when the temperature of the Universe was 3000 K, but have been redshifted by the expansion of the Universe to 3 K. The detection of these photons provides strong evidence for the Big Bang!
Is the Big Bang Model right or wrong?
Correct Prediction of the Big Bang Model:
So any new model will also have to predict these two things. As a result, the Big Bang model seems to be the best model....so far!