Cosmology
The cosmic microwave background radiation observed today is the most perfect black body radiation ever observed in nature, with a temperature of about 2.7K. It is a "snapshot" of the radiation at the time of decoupling between matter and radiation in the early universe. Prior to this time, most matter in the universe was in the form of an ionized plasma in thermal equilibrium with radiation.
According to Kondepudi and Prigogine, at very high temperatures (above 1010K; such temperatures existed in the very early universe), where the thermal motion separates protons and neutrons in spite of the strong nuclear forces, electron-positron pairs appear and disappear spontanteously and are in thermal equilibrium with electromagnetic radiation. These particles form a part of the black body spectrum, in addition to the electromagnetic radiation.
Read more about this topic: Black-body Radiation