I came across this post via Google (shocker right?) when I had the same question. Turns out, after 4 hours of reading through the very large thread (incredibly entertaining by the way - I suspect there might be trolls among us), we don't know the mechanism of how yet.
My first thought was, it seems to be the opposite of how air moves from areas of high pressure to low filling the voids. Nature seems to like equilibrium. Seems the larger universe doesn't always play by the same rules we see here on Earth, or more likely has rules we haven't quite worked out yet.
The math involved here is lightyears over my head so I appreciate the efforts of the experts to dumb things down a bit where possible. My brain can handle the concepts just not the math that gives you an answer.
Thanks to Incendia for the original post and all those that spent time on the discussion.
