Cosmic Microwave Background Radiation.

"When the universe was young, before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, stable atoms could form. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since the exact same photons fill a greater and greater universe. This is the source for the term relic radiation, another name for the CMBR."

http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation