Is the speed of light constant between galaxies?

For all other objects this is correct.But, Einstein's great insight was that this is in fact not the case for light.Every frame of reference will measure the same speed of light in a vaccuum and there is only the one value that can be measured for that speed all regardless of any frame. Edited by AZPaul3, : clarity - I hope

Oh. I can see that. I are just so good at comoo ... commo ... at reading.You are right, however. "c" is about as constant as you can get. Because c is constant in a vaccuum, everywhere, at all times, from every frame, it does let us build the needed transformations between units is differing systems. Whether we measure in kilometers per second or pinars per mosax we will achieve the same value when the units are properly transformed.What the OP seems to be missing is that light follows the spacetime curvature created by the presence of matter whether normal or dark. Everything follows that same path, even neutrinos, just not as fast as photons.I think he is trying to take the concept behind gravitational lensing to an unevidenced extreme.Edited by AZPaul3, : I will now stop second gussing my proofreading leave this as it is warts and all.

I yield to the details.