It could also be much smaller below the planck scale, but not causally connected to our level. That is, nothing in known physics prevents their being entire universes of particles that are so small that it is impossible to ever detect them with any of the Standard Model particles.
Right, can we reasonably rule out that such interactions don't happen in special circumstances that are hard for us to measure? Like in stars or black holes?
(Or randomly and very seldom but often enough in vast interstellar space to give rise to dark matter like phenomenon?)
We can't rule out the existence of something like this, so of course we can't rule out interactions with something we don't even know exists. But there's an infinity of things we can't rule out, but for which we don't have any reason to believe might exist, so it's not very interesting to speculate about them.
