That's a bit summarized, but the universe's mass is about 70-90% this black mass. Imagine a rock and a river. If you throw into the river a rock, that rock will create ondulations on the waters' surface. Now, think about the same but with planets. Planets are the rocks and the black mass is the water of the river. The planet has a mass, and every rotation or traslation movement, creates an ondulation on the black mass.
This was an Einstein theory that couldn't be proven till a couple of months ago.
So, this ondulations could be useds to move over the space, if we artificially create a big and strong enough wave with a new ship system, and we surf that wave.
The problem is that you have to exactly take the wave in a precise point or it wouldn't works; not too far and not too near from the wave. Exactly at the point, what's is supposed to be hard enough.