Board Thread:Questions and Answers/@comment-5157293-20131110013820/@comment-1713281-20131111150240

ProfessorKilroy wrote: What about Malekith and the dark elves, when they arrive at Asgard? They take their ship. It's possible they could all be on the same plane of existence, with the nine realms connected through Yggdrasill in another way, and the Bifrost just transporting across vast space.

It was stated by Thor that in the lead-up to the alignment of the 9 realms, the borders between them often become blurred and it becomes possible for inhabitants of the realms to move between them (such as Foster being able to move into...whatever realm it was that carried the aether). I know that likely sounds like a dues ex machina, but I didn't write the film.