Why does the red shift that we observe require the expansion of space?
The following was my lightbulb moment that helped me figure out why the redshifts had to be due to expansion and not velocity through space/time.
Very distant galaxies are heavily redshifted. If this was due to a real velocity through space/time it would take a massive amount of energy to produce this velocity. There really isn't anything to push on when it comes to a galaxy. It would be like pushing on a rope, or pushing on fog. You would need an absolutely massive gravitational force that is somehow spread evenly throughout the galaxy so it can keep it's spiral shape. Any massive object big enough to create these velocities would distort the galaxy beyond all imagination because the side closest to the galaxy (in the direction of the velocity) would experience a much higher gravitational force than the distant side of the galaxy.
On top of that, it would seem like a rather large coincidence that we are sitting in the middle of the show, so to speak. Redshift is the same wherever we look. It is only dependent on the distance between us and the galaxy. If these large redshifts in distant galaxies were due to a real velocity through space/time why don't we see massive redshifts in nearby galaxies, or even galaxies approaching us with a huge blueshift?
So a real velocity through space/time seems to be out of the question. It has to be something else. That something else is the expansion of the space between us and the galaxy.