So the bubble forms within, not outside? Because I've heard it both ways before, and even in the article it mentions something about two bubbles that are separ

So the bubble forms within, not outside? Because I've heard it both ways before, and even in the article it mentions something about two bubbles that are separate from each other colliding. In the later circumstances... What I can't understand is how two universes that are "separated" from each other by a vacuum (no space-time fabric) could ever collide... As they are separate by a pure nothingness, they for all purposes exist inside different realms and have no chance of ever colliding as there is nothing for them to travel through to reach each other.