I’m betting on primordial black holes for at least some of it. If you consider early universe densities their formation seems almost certain.
Most models predict that they exist but the unknowns are around how common they are and their mass ranges and therefore whether they could account for most or all dark matter.
There’s a little bit of speculation that “planet nine” is a PBH. I really hope for that one. It would give us actual access to a black hole within probe range, which could allow us to “complete physics.” Could also be a gateway to the universe through gravity assist, letting us yeet interstellar probes out at incredible velocities.
Most models predict that they exist but the unknowns are around how common they are and their mass ranges and therefore whether they could account for most or all dark matter.
There’s a little bit of speculation that “planet nine” is a PBH. I really hope for that one. It would give us actual access to a black hole within probe range, which could allow us to “complete physics.” Could also be a gateway to the universe through gravity assist, letting us yeet interstellar probes out at incredible velocities.