What it comes down to is that you’d need an energy field that you use for motion. But if you want a faster vector force, you’re going to need something that exerts more relative force. By definition, dark matter is a non-starter (it literally only interacts with conventional matter gravitationally). Gravity assists are already our best actual use case:
And if anyone is actually cruising around the galaxy, I think light sails are an excellent example of this.
Also, if anyone’s gotten really good at interstellar travel, they’ve figured out how to exist stably on timescales spanning at least tens of thousands of years. Our Achilles’ heel may not be speed so much as patience.
