By analogy to a high-bypass turbofan engine, which achieves high efficiency by giving a small push to a large quantity of inert air (or a wing, which gives an even smaller push to an even larger quantity of inert air), if one discovered a way to push/pull dark matter around, that could be used to achieve vastly more efficient space travel by having all this extra inert reaction mass that you don't have to carry with you. Energy is quadratically related to velocity, but momentum is linearly related to velocity, so you get a lot more momentum per unit of energy when you have a lot of mass to push around at low relative velocity.