The CMB Black Body Spectrum as Measured by COBE
The term black body refers to any object that is a perfect absorber of electromagnetic radiation (in other words, light). While no electromagnetic radiation passes through or is reflected from a black body, theory predicts that the body will emit radiation at all possible wavelengths. The amount of energy radiated is directly related to the temperature of the black body and its emission spectrum has a characteristic tapered distribution.
Measurements of the CMB indicate that it possesses a nearly perfect black body spectrum. While expansion stretches the wavelength of light emitted from the surface of last scattering, it also lowers the photon density, and thus intensity diminishes. These two effects cancel out so that the characteristic black body distribution is preserved.