In 1964 Arno Penzias and Robert Wilson discovered a form of radiation found everywhere in the universe. This data was later theorized to be connected with the Big Bang Theory. Penzias and Wilson noticed a radio background source all across the sky. It was later discovered that the sky is uniformly lit up in a certain radio frequency. The nearly unvarying nature of radio source is a key foundation in the cosmological principle.
The cosmological principle basically states that physics we can observe is uniformly applicable across the universe. The presence of this radio background literally everywhere lends credence to this theory. While initially observed as a radio source the phenomenon is caused by background radiation.
This background radiation is explained to be left over radiation from the start of the creation of the universe, better known as the Big Bang. In this light, this cosmic background radiation is also known as relic radiation – a relic of the beginning. This uniform background radiation has skyrocketed into the heart of any theory that seeks to explain the creation of the universe. Every theory has to account for the presence of this radiation. Another challenge is that this radiation though constant has slight fluctuations, these fluctuations has to be accounted for in other creation theories.
Cosmic Background Radiation Illustrated