I can guarantee that on none of my star charts was a planet named Gallifrey noted at those coordinates, nor did any well-worn and well-studied tomes ever mention your race. Asgard might have only held real power within the Nine, but it was not a kingdom ignorant of what existed outside of its grasp. I doubt the omission was in error.
Multiple dimensions had never been a theory I was fond of, however, it has never been one I entirely discredited either. The theory of spacial relativity is quite compelling, though I admit I am slightly out of my depth with the specifics.