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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 ...

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Interesting but light too effected by gravity, as it moves opposite to gravity it's wavelength length decreases but it is negligible. And I'm sure that light is effected by gravity because light cannot pass through a blackhole due to its high gravity. If light is not effected by gravity then it can pass through a blackhole and we can see inside a blackhole. ...

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Most of the light energy in the universe is still in the cosmic microwave background (CMB). Spring 2011 UC Berkeley Physics 250 class materials calculate from the fact that $T=2.73$ for the CMB: It follows that photons contribute only $0.0000485$ of the closure density.

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