There's something I don't quite understand about the 9 realms. Are they parallel universes, or just planets? Does Igdrasil resemble one universe or multiverse? If Malekith, and I'm taking info just from the movie, wanted to bring the universe back to darkness, did that meant that he would have wiped out Thanos, krees, skulls, etc? even Galactus, Celestials? More over Ragnarok its like this end of everything in fire, but it kind of effects only the norse gods and maybe earth, when it should effect all 9 realms, and by doing that, would everyone else above stated perish? No the other thing is Galactus, Celestials, etc are more powerful, then WTF? are they outside of Igdrasil? other dimensions? Are they just gonna sit around and say, bah just a little heat, not that bad.

I would say that most of the 9 realms exist in pocket dimensions. Asgard has been portrayed as sort of like being a giant asteroid drifting in space from afar with the rainbow bridge connecting it to other realms. So maybe the rainbow bridge acts as the means by which you can leave that pocket dimension for one of the other 9 worlds. They do seem to exist outside of the means of humans to perceive them with standard technology. I would also say that Ragnarok only effects the 9 realms and not other parts of the MU.