Three main observational results over the past century led astronomers to become certain that the universe began with the big bang. First, they found out that the universe is expandingmeaning that the separations between galaxies are becoming larger and larger. This led them to deduce that everything used to be extremely close together before some kind of explosion. Second, the big bang perfectly explains the abundance of helium and other nuclei like deuterium (an isotope of hydrogen) in the universe. A hot, dense, and expanding environment at the beginning could produce these nuclei in the abundance we observe today. Third, astronomers could actually observe the cosmic background radiationthe afterglow of the explosionfrom every direction in the universe. This last evidence so conclusively confirmed the theory of the universe's beginning that Stephen Hawking said, "It is the discovery of the century, if not of all time."