Could there be something in between the protons and neutrons in an atom? Is it just a completly empty space, or could there be another, smaller subatomic particle there that we can't see yet?

there is a vacum, if there is something even smaller than that then we cannot see it, cannot detect it and therefore it isnt there as far as we are concerned.

 

in all reaction thouhg, there is no sign that this is the case.

the space is only occupied by Nuclear forces, a bit like when 2 North North poles repel as if being obstructed by some elastic rubbery substance, when there`s clearly nothing but fresh air between them. the same applies inside the open space between particles in an atom.

 

Over simplified I know, but the point`s valid

Could there be something in between the protons and neutrons in an atom? Is it just a completly empty space, or could there be another, smaller subatomic particle there that we can't see yet?

 

You're thinking of them as particles. But they have a wave nature, too. You're in a regime where "between" doesn't have a lot of meaning.