If the universe is expanding now, then sometime in the past it was very, very small. It was discovered in 1965 that back in this early time it was also very hot…. Two radio astronomers, Penzias and Wilson found that the Universe is filled with microwave radiation with a blackbody spectrum of temperature 2.7K.
Together these led to the picture of the origin of the universe in a hot Big Bang! As the universe has expanded it has cooled off...
COSMOS to HUMANITY, Spring 2008
Lecture 2 , slide 12