The Aetherium is simply another name for space. It's quite vast, as you might imagine, and possesses currents and winds of its own. [It's almost like explaining to a child who has no concept of the nature of the universe, though there's only a hint of that in her voice.]

Again, though, given the distances needed to be covered, it's far more efficient to use solar-powered ships rather than relying on the often unpredictable natural forces.