Background radiation gave the universe a history
The cosmic background radiation, which was discovered in 1964, was important proof of the Big Bang theory. Suddenly it was clear that the universe is neither static nor has an infinite past, rather it has a beginning and a history.
Radiation is a relic from the young universe
The cosmic background radiation is consistent with the Big Bang model's account of a hot newborn universe which later cooled as it expanded. The radiation is very uniform from all directions, and the minor differences can be explained by the universe expanding extremely fast immediately after the Big Bang.
The universe grew irregularly
Since the Big Bang, the universe has grown in three limited epochs: an ultrashort period of explosive growth (inflation), a long period of stable growth, and an equally long period in which the expansion accelerated.