.  The doppler effect is real. A moving source that is going away from an observer will show a shift to lower frequencies (red end of the spectrum so to speak). I would expect that the Galaxies are expanding and generally rushing away from each other. They should be slowing now in their expansion due to the effect of what we call gravity.  Instead they appear to be accelerating which would suggest a force or energy that is somehow emanating from free space. This is what defies our current understanding and prompts one to think about the possibility of another explanation or that the original premise of acceleration is false. For my money a better premise would be that there is another much more massive , encompassing universe just beyond our vision that is attracting our universe due to gravity.  