The !kaf have all that. Nobody else. It works (or did, when it comes to the Stargate gates, but nobody can figure out how. I like the idea of having one species out there with Clarke's Law-level tech.

Even if no-one in the setting knows it works, *you* have to have some idea how it is posisble, or you shouldn't call it a hard science setting. If you use negative mass, you can in fact have stargates and effectively inertialess drive without violating physcial laws as currently understood.