For instance, faster-than-light travel is impossible under any current scientific theory, yet is a staple of space-faring sci-fi.
If Faster-than-light travel is necessary for your story, you can name it ('the Jump' or whatever) and then just use it.
Their merchandise is information, and it is from the Outsiders that mankind gains the knowledge of faster-than-light travel.
Most ships in the Star Trek series are capable of faster-than-light travel using warp drives or similar propulsion methods.
Some Generation Ships are discovered, still in progress towards their destination, by faster-than-light ships, either with crew intact or crew all dead/missing, and perhaps a mystery to solve.
