Black holes are regions of space in which gravity is so strong that nothing can escape, not even light. Conventionally, black holes form during a gravitational collapse, after a large supernova for example.
But there is another class of objects called primordial black holes that cosmologists think must have formed in a different way. These are essentially leftovers from the hugely dense ball of stuff from which the universe expanded, some parts of which must have been dense enough to form black holes.
These primordial black holes would then have been widely dispersed as the universe expanded.
Primordial black holes are very different beasts to the ones that form when stars die, in particular because they ought to be much smaller.
Although nobody has yet seen a primordial black hole, our knowledge of them comes from thinking about the processes that must have occurred shortly after the Big Bang.
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