Ah yes about the Cosmic Microwave Background (CMB). The theory goes that this is the last scattering surface for photons before the universe become transparent to them which is when electrons recombined with ions. This surface radiates like a blackbody (basically the same radiation spectrum as the Sun, people, the Earth, buildings, basically anything with a temperature). Orginally it radiated at a Blackbody Temperature of about 3000 Kelvin, with the universe expanding it appears to cool and now is at 2.7 Kelvin. The variations on this Blackbody (anisotropies) can tell us how matter was distrubuted at the epoch that the CMB was formed (known as recombination) which is useful for pinning down cosmological parameters (i.e. how much dark matter there is, whether the universe is flat or not, whether or not there is cosmological constant).
