The cosmic microwave background is a uniform 2.7 Kelvin radiation that permeates the entire universe. Although it was postulated almost 50 years before, Penzias and Wilson discovered the cosmic microwave background accidentally in the 1970's. Working at Bell Labs, these two scientists were using a radio telescope to observe distant stars. They found, however, that no matter where they pointed their telescope they observed an approximately 3 Kelvin background signal. After convincing themselves that this signal was real and not some artifact of their instrument, they consulted with a team at Princeton University that had been searching for the cosmic microwave background. The Princeton team confirmed what Penzias and Wilson had found. Apparently, Penzias and Wilson had accidentally stumbled upon the oldest observable in the entire universe. Why does the cosmic microwave background exist and permeate all of space? Just an instant after the Big Bang, all matter in the universe was so energetic, or hot, that it existed as free particles known as "quarks." In the fractions of a second following, the universe expanded and cooled until the quarks lost enough energy to form electrons, protons, and neutrons, the building blocks of ordinary matter. Photons, the smallest particles of light, also filled the universe and were so energetic that they "bounced" off electrons, keeping the electrons and protons from forming atoms. After approximately 400,000 more years, the photons lost enough energy that atoms could form readily. Without any lone electrons off of which photons could "bounce," the photons began streaming unimpeded all through the universe, mostly unchanged but for one exception. Due to the further expansion and cooling of the universe, these photons have cooled to just 2.7 degrees above absolute zero. It was these same photons that Penzias and Wilson observed approximately 13.6 billion years later here on Earth.