I feel like I need to let a lot of people know that this "Aether" thing is a very tricky subject.
Treyarch mean it to be a place a bit like purgatory. Neither real nor non-real. It's in the background, underlying everything that happens.
However, the entire story (of zombies) is also heavily focused on sci-fi physics. E.g. Quantum Mechanics being able to create black holes at will and things of that nature.
Backstory: In the world of Physics a few years ago, scientists didn't understand how gravity worked (and they still don't - not really), so some scientists created a theory. This theory was that there was an Aether. A dimension that co-existed with our 3 spacial and 1 time dimension(s). In this dimension there were particles and fields that moulded around our own, making interactions like gravity possible where it isn't in the real world.
My point: After only a couple of years this theory was disproven. It just didn't work right. So we created the idea of fields (gravitational, Higgs, Electron, Proton, etc). These fields are everywhere and particles are just where a lot of energy builds up in a certain area in one of these fields. But that doesn't matter.
My point is that this "Aether" was disproven. So what, exactly, does Richtofen refer to when he talks about the Aether?
Note: My science isn't perfect. Some of it may be wrong (e.g. it might not be gravity that was affected in the Aether) but the general ideas are 100% correct. There is a disproven Aether theory in Physics.
My conclusion: There are only so many possiblities here. Richtofen could've found a different dimension that he just called the Aether. They could be in an alternate universe where the Aether exists. I honestly don't have a conclusion. Like ever, Treyarch don't give us the full story and so there are too many answers to 1 question.
I just thought I'd let you know that the "Aether" might not be as simple as the Aether mod in minecraft :P