All mass produces time dilation, and a reduced velocity of light isotropically, and for distant mass the effect is proportional to the inverse radius. In addition there is a radial reduction in coordinate velocity, but this is direction and does not add in the same way as the time dilation component. If mass is approximately distributed evenly, then the amount of mass in a differential radius increases proportional to R^2, so distant matter dominates the effect unless one is very close to a particular gravitational source. So how much would light speed increase if the known distant mass, including dark matter, were removed? I could do crude calculation, but there might be many factors I wouldn't think of. I'm looking for an authoritative paper. For example, if light speed would quadruple without this mass, relative to some even more distant observer (since we would not notice the difference, being local observers), then one could say the fraction attributable to the distant mass was something like (1 - 1/4) = 3/4, just as an approximate gauge.

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