Decaying gravity would answer much, especially if it decreased very fast long ago. And continues to decrease at a very slow rate. Forever. I would highly suspect this dynamic to be so.
And this does not explain rotation within a galaxy. But the trajectory and structure of the orbits can, I believe. The circumference of those orbits are assumed to be arcs, and the velocity, the length, the period and the momentum is calculated from such. But I believe the orbital circumference is not an arc, but is a helix. And therefore the amplitudes and directions of the calculations are very mis-leading. The velocity and momentum are constantly changing, and only a portion is giving you the right observed direction. An orbital helical trajectory has not been discerned. But can be observed with Io. And now finding a debris torus for all planets confirms this to me.
I do believe that our observations and measurements are and have been very mis-leading. We need to measure Z motion, along with X and Y motion. And that Z motion will be angular, not linear. An orbit has two perpendicular angular accelerations, not one.
A portion of the velocity and momentum is not in the direction of the arc, it's perpendicular to the arc. Giving more velocity and momentum than seened and measured.