The cosmic microwave background radiation was first detected by physicists in 1965. The radiation did not seem to be originating from any particular location in the sky but was uniformly spread out. Using radiation from black bodies, scientists were able to place the temperature that could result in the radiation at 3.5 K
A search for the cause of this radiation revealed that it is a remnant of the Big Bang that led to the creation of the universe. As the universe expands, due to the Doppler effect all light that we observe is stretched and there is a reduction in its frequency. The earliest light emitted when the Universe was just created has been stretched out over billions of years to the frequency of microwave. The uniform expansion of the Universe makes the microwave appear as cosmic background radiation. The detection of this radiation provides further proof for the Big Bang Theory, which postulates that the Universe originated from a single point, 15 billion years ago, in what can be called a giant explosion and is commonly referred to as the "Big Bang."