The cosmic microwave background (CMB) is electromagnetic radiation left over from the hottest phases of the early universe. This ‘afterglow’ of the Big Bang fills the entire universe, and it has almost the same characteristics in all directions.
However, satellite observations have revealed tiny irregularities in this radiation. These irregularities reveal information about the very first instants of the Big Bang - so-called 'quantum fluctuations'.
As the universe grew in size, these tiny fluctuations in temperature expanded to cosmic dimensions.