Spiral Galaxies Rotate. Elliptical Galaxies do not rotate. Why?
I believe the conventional assumption concerning angular moment in the universe on very large scales is that it is net zero. (i.e. The universe does not rotate.) As far as I know there is no observational evidence on a very large scale or on a large scale to question that base assumption.
For example a cluster of galaxies does not rotate. If clusters of galaxies did rotate as a group that would be evidence that the net angular moment in the universe was not zero.
I have not seen a good explanation as to why the stars in a spiral galaxy rotate as a group which means that a spiral galaxy has net angular moment.
Just as puzzling for the opposite reason, is why the stars in an elliptical galaxy do not rotate as a group. The stars in an elliptical galaxy orbit the elliptical galaxy's centre such that the net angular moment of the entire elliptical galaxy is zero.
Why do the stars as a group in a spiral galaxy rotate? Can angular moment be created?
Also curiously, elliptical galaxies have almost no gas and dust, whereas, spiral galaxies, I believe, have 20% of their mass in gas and dust.
The diagram in the attached link shows how the net angular moment of spiral galaxy is positive while it is net zero for an elliptical galaxy.