Dark matter a medium for light propagation Is dark matter a candidate to fill void left by luminiferous ether as a medium for light travel?
 A: No.
There is no void left by the lack of an aether. The very notion of aether should serve as a warning as to how catastrophically analogical reasoning can fail. "Water waves are in water, sound waves are in air, therefore there must be something in which light propagates." This is flawed logic, and decades of physics were arguably hindered by adhering to it.
In fact, any material medium for light would contradict the beautiful result of Michelson and Morley, showing that the speed of light does not depend on velocity with respect to some material's frame. This invariance is in fact now at the very heart of modern physics, and is the basis for relativity, which has been verified in innumerable experiments.
Dark matter is, according to the leading theories, some form of matter that is basically normal except that it essentially doesn't interact via the electromagnetic force. As such, it is actually a poor candidate for explaining anything to do with light, even if there were something that needed explaining.
