Dark matter, actually.
Suspending the more obvious issues to do with UFOs, the minutiae of their propulsion systems are irrelevant. Every vehicle's operating principles are blindingly clear to any observer with enough understanding and familiarity with it, and even the most mundane bit of today's technology would appear magical in the not too distant past.
We'll be looking at it from afar and applying the filters of our understanding and experience, which are pretty lacking. Except as a starting point for a brainstorming session at JPL or DARPA, none of it matters.
Personally, I'd be more likely to attribute unknown phenomena in the sky with the Pentagon than with aliens.