Remnant heat from the "big bang," first detected in 1965, is in the form of microwave radiation because the universe has cooled considerably since its birth. But the universe acts as a perfect emitter and absorber of radiation, and for this reason it is called a "blackbody" as all objects are which emit and absorb perfectly.

Our Sun is almost a blackbody, so we'll use it as an example of what to expect from the much cooler universe. First, the Sun has a dominant color, yellow (corresponding to a certain wavelength), and this color is associated with a certain temperature, about 6000 K. The microwave radiation remnant (Cosmic Microwave Background, CMB) has a dominant wavelenth (1 mm), and this wavelenth is associated with the temperature 2.7 K. Another blackbody property of the Sun is its emission of radiation of many colors (as seen through a prism) as well as heat (infrared) and ultraviolet radiation. This range of wavelengthts is easily detected because the Sun is comparitively hot, but the CMB also creates a range of microwave wavelengths consistent with a blackbody of much lower temperature.

The FIRAS instrument on the COBE (Cosmic Background Explorer) satellite measured this range of wavelengths and found them to be exactly as expected for a blackbody, as shown below.