Chloe Bennett also later retracted her claim
, for what it's worth (perhaps not sincerely, though). Even if Ms. Bennett has no inside knowledge of what it is, however, it's hard to imagine it being anything else. Either way, though, I think it's clear that Lady Sif does not have full knowledge of everyone who has reached Earth.
I don't know how Peter Quill gets into space in the comics. However, one report for the movie says:
I don't think Chitauri space is another dimension. Whether the Nine Realms are or are not is the big debate (I don't personally think it changes much either way). The main view, however, is all of known space is in Midgard, while the other eight realms are in other dimensions. That means the Chitauri are in Midgard. The portal that opened that gave them a shortcut, iirc from the Avengers, was created with the Space Stone by Loki. I don't think it was a portal to another universe, unless there's something from the comics suggesting that the Chitauri are not from normal space.