Stephen Hawking and Bernard Carr hypothesized that conditions close to the Big Bang would have produced primodial black holes, including some that were smaller than can be formed by collapsing stars, and some that are bigger than is easily explained by stellar collapse black holes combining over time.
And at some point, there was a school of thought that the unexplained "missing" mass in the universe might actually be caught up in black holes that we are not able to detect. This idea lost popularity, in the scientific community, as people explored ideas like Weakly interacting massive particles (WIMPs) but after decades without seeing signs of the required fundamental particles, some are investigating the idea again. Some recent evidence from gravitational limits the possibilities, but some researchers remain hopeful that the explanation for the universe’s "missing" mass is tied to black holes rather than exotic particles.
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