Could Dark Matter be used as reaction mass for a propulsion device? Assuming a best case scenario, where humans are able to discover a way to interact with Dark Matter, could we use dark matter like a row boat uses the water?
Assuming Dark Matter is made of WIMPs, they are going to be moving with respect to a propulsion device, but assuming your device was able to accelerate the particles in a specified direction, could this device solve the problem of exponentially increasing rocket weight in relation to Delta-V? is the density of Dark Matter in our galaxy high enough to provide stuff to push on?
It would be pretty cool since it would be like water, but only interact with your engine, passing through your ship, unaffected.
 A: Assuming there's a way to put a force on dark matter particles, then it's just an application of Newton's third law. Airplanes push air backwards to move forward an down to move up.
The problem I see is that any vehicle that interacts with dark matter to a significant degree will experience a drag force from the dark matter particles that it doesn't accelerate. This spaceship won't be able to coast through space, but will require constant power to maintain a constant speed with respect to the local part of the galaxy--just like an airplane or boat experiences drag in air and water.
A: Maybe, but you have various real issues to get over, and it could be that none of them have a solution. Since we don't know what dark matter is, even if it is wimps, we don't know how to interact with it. And even with normal matter, interstellar propulsion is still an unsolved issue. See the summary of the various techniques that have been thought of with regular matter. See https://en.m.wikipedia.org/wiki/Spacecraft_propulsion
As you can see, there is no panacea. One of the currently 'hot' topics using normal matter is ion propulsion, and it still is not great, you still need material to ionize and spew off. Not too much material in interstellar space, where the density of matter can vary from about 0.1 atom/$cm^3$ to maybe $10^6$ atoms/$cm^3$ In some des se areas. Air is $10^{19}$, for comparison.  
Dark matter has lots of disadvantages and a few advantages, conceptually. Advantages are that it has a factor of about 7 greater density than normal matter, there is more. But, it's not a major factor, you still will need to carry some of it, maybe too much as you won't find that much between stars and between galaxies. They tend to concentrate more around galaxies and you still don't have that much in the interstellar and intergalactic medium. 
A negative factor is that you can't use electromagnetic propulsion, as dark matter does not interact electromagnetically, no charge. It also has no strong force interactions. So you have gravity and weak forces left. For gravity you'd have to somehow build a black hole, highly dense but not too massive, to serve as your engine, and figure out how to do it so you create large impulses for the dark matter so you can go the other way. I don't know how to do that. The other possible force you can use is the weak force, you'll have to figure out how use it to create the dark matter impulse also. 
But presumably, if you figured out how to do one of those two engines,  you could make your weak force or gravity engine of dark matter stuff, while the rest of your ship is normal matter, and you won't have your ship dragged back because of friction with dark matter. 
Still, all of this is highly hypothetical. It may be that none of those can be done. 
For normal matter NASA is exploring various ways, with ion propulsion one of them.  
