Some theories of the expanding universe say that eventually the universe's expansion could be slowed and eventually pulled back in on itself. My question is why can we assume the edge of the universe is effected by gravity? Does this barrier necessarily have to be made up of a physical substance? Also this might be a weird question but what do yo think would happen if I flew up next to this expanding barrier and tried to put my hand through it? Would I lose my hand or possible my hand wouldn't penetrate it.