The cosmic microwave back ground radiation is important evidence for the Big Bang. It was discovered by accident by two radioastronomers, who eliminated all significant errors and noise from their apparatus but were still left with a signal characteristic of a black body source at around 3 K. The signal did was virtuall the same in all directions and not seem to be associated with any single source.
The early Universe was very dense and hot, and rapidly expanding and cooling. While very young radiation could not travel through it, because any photon would be repeatedly absorbed and re – emitted. There came a point however, when the Universe cooled and became transparent to radiation. The cosmic background radiation we have now is basically a photograph of the Universe at that point when it became transparent to radiation, redshifted from the wavelength corresponding to the temperature then to a wavelength corresponding to a temperature of about 3 K. A map of background radiation shows the distribution of matter at the point when the Universe became transparent: the image below is enhanced to emphasise the difference. In fact the density at this point is the same everywhere to about 1 part in 10,000.
