What if dark matter turns out the key to a decent space drive? There appear to be galaxies without it (a good reason for believing it exists where there is evidence) and so any aliens there would be SOL.

Astonishingly enough, you might not be far off. That is the operating principle behind the Alcubierre Drive. https://en.wikipedia.org/wiki/Alcubierre_drive

Edit: Apologies, it seems it does not in fact use dark matter.

Unfortunately, Alcubierre doesn't use dark matter. It uses negative mass (or even imaginary mass) matter. Dark Matter is believed to have regular positive mass (if it were negative mass, we'd be wondering why galaxies seemed to have less matter than they should).

The existence of negative mass exotic matter remains an open question.

The Alcubierre drive relies on negative matter, not dark matter.

No doubt stolen by the untrustworthy Andromedites!