The Rift(s)?

[SilverArrow20XX]

The Rift is just interdimensional space.
Omega didn't create it. He merely created a space within it.
It's like Hyperspace.
You have to move through it to get to other dimensions, and it can also be used to freely move through time and space.
Bad guys also love to just hang out there so that they can have a safe space outside of existence.

The Aetherial Rift I think is just a thin Rift between the Shards, that are close enough to be touching each other in some spots. Basically just dipping your toes into the Rift, so non-corporeal beings like the ascians can cross there, even if diving into the deep end is probably out of their reach without aid.

Outer Space is The Great Expanse.