Hawking's Theory of Black Hole Formation

The earth could never actually become a black hole, because it simply doesn't have enough mass for gravity to collapse it into that infinite density. What Stephen Hawking and his colleagues realized back in the 70s is that there were actually times in the universe's distant past when everything was much, much denser. And so, during this time, something with a mass like the earths could have collapsed into a black hole back then. Today, people call these hypothetical particle-sized black holes that may have formed in the very early universe, primeval black holes.