Personally, I would have it turn out that what was first thought to be the Earth dimension was actually the distant past. That after Mother died, the resulting backlash eventually rendered the planet uninhabitable, forcing humans to abandon aether on an interstellar exodus where they encountered Xion. In other words, it turns out that Earth was the homeworld of the Photoners. It would add an extra layer of irony to Mother's revenge crusade because it turns out that she had a hand in creating her reviled creators.
How did you get in my head? This is pretty much my head canon, regardless what the official version may turn out. That's the only way to have any of it make sense and "feel" complete. It even serves the dimensional/karmic/looping theme they've got under the hood (i.e., Xion, Matoi, and I believe there is a case for saying WE (the player) are also Xion's child... who else would have the run of ORACLE FLEET and pretty much any ship or block they come to?).