Could dark matter be considered a medium? Long ago, it was theorized that light had to travel through a medium, until the particle/wave duality of light was discovered.  Then it changed to the idea that light can travel through the vacuum of space as a particle.  

Light travels at different speeds depending on the medium that the light is traveling through.  It travels slower in water than it does through air.  

However, is there any possibility that dark matter is a medium that light travels through?  Could it be conceivable that what we recognize as light speed is really just the speed at which light travels through this medium?  If we were to remove dark matter, would light go faster?
I don't see any reason why dark matter couldn't be considered a medium just as air or water are, but this does not mean that your conclusion is correct.

Dark matter does not interact with photons, however the reason we know dark matter exists is because of the measurable effect of its gravity.  Galaxies have a tendency to clump around it.

Light travels unimpeded at its standard rate between galaxies where we don't detect any dark matter, so interesting question though it is, I'm afraid the answer is no.