The Cosmic Microwave Background, or CMB, is radiation that fills the universe and can be detected in every direction. Microwaves are invisible to the naked eye so they cannot be seen without instruments. Created shortly after the universe came into being in the Big Bang, the CMB represents the earliest radiation that can be detected. Astronomers have likened the CMB to seeing sunlight penetrating an overcast sky.
The Cosmic Microwave Background
Source SPACE.com: All about our solar system, outer space and exploration
