What hypothetical black hole type could have formed soon after the Big Bang?

A hypothetical black hole that could have formed soon after the Big Bang is a primordial black hole.

Primordial black holes, or PBHs, are proposed to have formed in the extremely dense early universe, before the first stars existed. An unusually large density fluctuation could have overcome pressure and caused a region of matter, radiation, or energy to collapse directly under its own gravity.

That origin distinguishes PBHs from stellar black holes, which form from the collapse of massive stars. PBHs are not restricted to ordinary stellar-remnant masses: models allow possibilities ranging from asteroid-mass objects to much larger black holes. Their existence remains unconfirmed, although they have been studied as possible dark-matter candidates and as seeds for early supermassive black holes.

The idea was proposed by Yakov Zeldovich and Igor Novikov in 1966 and studied in detail by Stephen Hawking in 1971. A primordial black hole is not the same as a white hole, a hypothetical time-reversed object, or a boson star, which is a different speculative compact object.