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human
[MCU] (Thor Ragnarok and earlier spoilers) The world tree and the rest of the universe?

So In Thor The Dark World, we see that the 9 realms are converging and allow travel between the 9 without need to use the bifrost or any other means. At first I thought that maybe they were in different dimensions or something and that is what made them separate from the rest of the universe (the Guardians of the Galaxy type stuff). But when Thor calls his hammer back to himself in TDW the hammer flies up into space when he goes through one of the convergences, so it suggests that there is some straight line between the 9 worlds.

In Thor Ragnarok, Hala mentions that Odin stopped his conquering at 9 worlds while she desired to keep going until they ruled the whole universe. Which further suggests that these 9 worlds are in our universe. So my question is, what causes the convergence between the 9 worlds? The only thing that I can think that relates the 9 is the bifrost. So does the Bifrost cause the convergence? 
If Hala succeeded, and more planets were conquered would those be connected in the next convergence?

Edit: Similar question, are the 9 worlds key points in the universe? Thor says that Midgard (Earth) is central to the world tree which is why the convergence is the strongest there. And the Dark Elves seem to think that they can get the fastest coverage of darkening the universe by using the convergence. So is the earth some central point of the universe? Is that why Dormamu was so interested in using our planet to enter our dimension?
gpt
1. The Convergence predates Odin conquering the Nine Worlds, as it was shown during King Bor's attack on the Dark Elves over five thousand years ago.

2. The Convergence may be a side effect of some cosmic phenomena connecting the Nine Worlds. For this reason, the Bifrost was able to piggyback on this phenomena, which means that it cannot be used to travel to any other world. 

3. Odin probably stopped after conquering the Nine Worlds because taking other worlds that are outside the Bifrost network would take too much effort, considering how Asgard doesn't appear to have significant space navy capabilities.