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.
Assuming P9 is further out from the sun than the other planets... It would be orbiting slower than the rest, and will probably give you a smaller gravity assist than the rest. Gravity assist can only change your velocity by up to your relative velocity (because it's just a flyby orbit as far as the assister is concerned - speed in equals speed out).
This is different from a normal gravity assist. Close to a black hole the gravitational field would be quite strong, making this potentially very powerful.
You wouldn't get close enough to be torn apart. You'd fly by and thrust into the gravity well for an Oberth effect boost. I'm thinking of unmanned probes of course. We could send them on flyby missions to other star systems to do things like take pictures of possibly habitable exoplanets. It's unlikely that humans could survive the acceleration of such a maneuver without ending up like this guy:
There wouldn't be much radiation unless the black hole were actively "eating" something, which a PBH in far solar orbit would not. That's part of what makes it hard to find and what makes PBHs candidates for dark matter: they're dark and don't do much unless something encounters them. There would be no net emission of Hawking radiation since the BH's Hawking temperature would be colder than the cosmic microwave background.
The hypothetical PBH that could be planet 9 would be about 2-3 Earth masses and about the size of a golf ball in terms of event horizon radius. You couldn't get that close to it without being "spaghettified" and added to its mass.
Probably the most valuable thing we could do with it is study it and use it as a lab to learn about quantum gravity. We could chuck small things into it with probes nearby to precisely measure the result, etc. All black holes we know about are far too distant to reach. Having one we could study would be huge.
I think primordial black holes are the Occam's Razor solution too. No exotic new particles required, no modifications to gravity required, no new physics period. Observational results have ruled out certain masses of black holes but far from all.
Primordial black holes do not require new fundamental physics, but for them to constitute the primary component of dark matter would require a revision, at some level, of our narrative of the history of the universe. This gets a bit outside of my core knowledge, but as it stands there aren't any super solid mechanisms for generating black holes of a plausible mass distribution, early in the universe, such that we would see what we see today.
But, IMO, this is worthy of more study both theoretically and experimentally. An update to the evolution of the universe would be awesome!
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.