Well, whether “darkness” has speed I think depends on the reality of dark matter. If there really is dark matter, like 98% of the universe as some day, then there is a speed of dark, kind of. And because dark is just the absence of energy it’s as fast as light.

Btw, what’s that experiment when they slowed down light? Can’t remember and too lazy to google it. IBM did it I think.

Gravitation itself doesn’t have a speed, since it appears to fill the universe, but gravitational waves have speed, even though they are not yet directly proven, right? Only indirectly, but what was really seen then I wouldn’t bet the farm on.
When is LISA bound to take off?