I have my own little thoughts on how intergalactic travel might be achieved, and basically, once you think through it, it comes down to time travel.
But not in the classic sense.
If all UFOs are actually time machines, the Roswell crash would have never happened.
I think its possible that some are time machines.. but I think many of them are experimental aircraft (search for Xcraft). I think that very few of
them today are actually alien craft. The aliens are already here, they have no need to travel for the most part.