I've always assumed dark *anything* was nonsense.
"Dark" just means "we can't see it". Anything that isn't either emitting light or other electromagnetic radiation, or reflecting light from something it's close to that's emitting light or other electromagnetic radiation, is "dark".
The data is, if you look at the motion of the stuff we see, and calculate the motion based on the masses of stuff we see, the trajectories deviate. The obvious thing to guess is that it's the gravity of something (or things) we don't see. This has worked before-- for example, this is how Neptune was discovered; it's how close binary stars were discovered.
An alternate hypothesis is that there's some other force pulling or pushing on them, or else gravity doesn't work the way we think it works. Either way, you're hypothesizing something unknown.