So what happens to anything trying to transit the boundary?
1) There is a rule, or metarule, that governs the transition. But for this to be the case the metarule must apply on both sides of the boundary, and be consistent with both sets. This means that the "local rules" on either side of the boundary are just manifestations of the metarule, and that we are dealing with what physicists call a "phase transition", (the sort of thing that happens when ice melts) and not a genuine rule change at all.
2) There is no rule, and what happens on one side of the boundary has no bearing on what happens on the other side. In which case "nothing" crosses over, and what we have is a place in "our" space where things happen at random. Noether's Theorem - If the Laws of Physics are symmetric through all possible rotations (ie if they are universal) then the existence of the universe must be underpinned by a universal constant (ie energy) - has implications here since it means there can't be such a boundary - a reaon for thinking that the universe is unbounded.