Could dark matter be considered a medium?
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.