Well in 2005 Hannestad did a paper in which Dark energy can behave as a liquid. So which Dark are you referring? Dark matter, Dark energy or No light?
At the time he also noted we had nothing sensitive enough for observations (kind of odd, seeing as he made some).
The paper itself was titled
Constraints on the sound speed of dark energy
doi.org/10.1103/PhysRevD.71.103519
Fast forward to 2010................
Roland de Putter, Dragan Huterer, and Eric V. Linder
Measuring the speed of dark: Detecting dark energy perturbations
They talk about measuring the sound from dark energy, while they dont give a figure they do note some things as the sound drops below the speed of light. SO straight away they are suggesting it is faster than the speed of light. But again equations given dont give a real speed, theoretical yes, real no. Also noted as you would expect that its speed alters the time frame as it passes the speed of light.
I dont have a Doi number for that paper but its the same journal of physics as the first.
Anyway long storey short its speed depends on which state it is in.
Most of those papers seem to use special relativity as a base in the equations, I am looking for the paper from last year, I didnt actually read it all. But apparently newer work throws holes into special relativity, and to me at least, points to areas that the Man himself, was looking into with the theory of everything. Maybe because he developed the theory of special relativity he boxed himself into a corner. I dunno I am no physicist, anyone who plays with a calculator and chalk all day, seems odd to me.