Sounds like something we could check empirically. Some cosmic rays are protons from outside of the solar system. We could capture those and compare them with lo

Sounds like something we could check empirically. Some cosmic rays are protons from outside of the solar system. We could capture those and compare them with local protons to see if they're actually the same kind of particle. (We can check if two particles are the same kind or not by seeing what happens if we try to put them in the same place. If they're both protons, they should refuse to be put into the same state due to the Pauli exclusion principle.) As far as I know the experiment hasn't actually been done, though.