BIG BANG THEORY
The whole idea behind the big bang theory dates back between 10 to 20 billion years ago. This is when it was thought that the universe exploded and matter was dispersed in all directions. George Gamow's thoughts about the big bang theory states that space first expanded through a very drastic temperature and pressure, but then eventually cooled down. Gamow was a man of a few words, he explained these complex and difficult theories in simple ways.
http://liftoff.msfc.nasa.gov/academy/universe/b_bang.html
George Gamow thought there would be leftover radiation from the big bang. Through his work he found out that what was left over might be detectable and so he used this way of doing that. First by finding the original temperature of the explosion and then figured out what the temperature reduction would be that was caused by the universe's subsequent expansion. He came up with a value of 5 Kelvins.
http://www.gwu.edu/~physics/gwmageh.htm