Gotta be mind-upload technology.

It's my favorite solution to the fermi paradox: We don't see aliens because once a species develops the technology to transfer their consciousnesses into a substrate more configurable and durable than the organic stuff nature made them, there's just no reason to ever leave your planet.

Second choice is some viable intragalactic transport, which would enable me to see a black hole in person. Perhaps even the big one in the center (though im told they all look alike ....)