Spiral galaxies are flat due to the accumulated angular momentum that exist in all satellite systems,which make up the spiral Galaxy.
A galaxy is not a satellite system as the outer arms are locked in step with the galactic center,like spokes of a wheel.
Cosmologists invented dark matter to stop them from flying apart but activity at the galactic center is the driving force of it’s evolution.
Due to the shrinking volume of a galaxy,when observed from the outside light is shifted to the red and this red shift is the same no matter which side a galaxy is viewed from
This is interpreted [incorrectly] as a recessional velocity
