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.
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.
