My theory is that beyond natural barriers, which exist all over the place, the laws of physics change dramatically. Dark matter for example, which is still a huge mystery, would be a form of matter (or matter like) that exists on the other side of one of those barriers.
You can pretty much observe that as your approach a barrier, the rules start to change. Matter travelling near the speed of light, matter near absolute zero. For example;
https://www.google.ca/amp/s/phys.org...tic-states.amp
Basically, in contrast to traditional beliefs that everything can only exist within these limits, I think that these are not insurmountable barriers. There may be entirely new states and universes that have totally different rules beyond them that we can't observe, and then more beyond that, and more beyond that... etc.
It actually ties in with a much larger in depth theory, but that is pretty fringe. I am not getting into that, lol.