After stealing the Aether from Malekith, where did Bor hide it?
Bury it deep. Somewhere no-one will ever find it
It's almost certainly Svartalfheim, former home of The Dark Elves, The Dark World itself.
It's a barren, desolate wasteland following the first war, making it the perfect place to hide something. There's also the fact that it resides in a star system that is ravaged by Dark Matter, something The Aether is known to produce, and that when Jane Foster found The Aether, the Dark Elves on Svartalfheim woke up. The link could extend lightyears across worlds, but I think it's more likely that it's because they're on the same planet.
You can also narrow it down to Svartalfheim via some brief logic that isn't quite concrete, but it makes sense:
Which leaves us with Svartalfheim.
Personally, I think it's the only logical choice. But if anybody can show something that disproves anything I've said, they're welcome.