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.
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@mariopasquato the total quantity of dark matter in the universe is 5.,5x baryonic matter though, and unlike baryonic matter the dark matter won't have unintended interactions with the rest of the ship.
@JonathanRay There is a lot of it but it’s not concentrated enough compared to baryonic matter in the places where we are likely to hang out (all the things we care to eat, fuck, mine, or otherwise interact with are likely made of baryons residing on planets made of baryons around stars made of baryons). Unless we come up with a process that has a huge cross section with DM I am afraid this isn’t going to work. Also, unrelated question, how does this resolve if DM does not exist?