Yesterday, I saw someone talking about the fact that the hypothetical Planet 9 of our solar system would be a primordial black hole, beyond Neptune with the size of a tennis/baseball ball that remains from the early days of the universe, and that this could also explain the dark matter because it would be something common in the universe, even if for the moment none could be found yet.

Yes there was indeed a recent article studying this hypothesis.

The nice thing with our era is that we have a lot of issues here and there (both in data and at the theoretical level). The motivations to work on new phenomena is thus quite important, and the absence of guiding hints for nature forces us to explore all possibilities (we need to be pragmatic). This is exactly why I work in this field! :)