# Kinetic energy and galaxy arm rotation rate

The higher than expected rotational velocity of stars and gas clouds in the outskirts of galaxies is explained today by invoking dark mater that supplies not only the additional gravitational mass, but explains the amount of gravitational lensing of more distant galaxies. Furthermore, dark matter distribution about spiral galaxies places it on the outside of galaxies and not so much on the inside.

Certainly the higher than expected velocity on the outside of galaxies also translates into higher than expected kinetic energy. Should the extra kinetic also increase the gravitational stress energy tensor in that region of space.

If so, do our models already factor in kinetic energy and it's gravitational stress energy tensor, overlook it, or the effect is just too minuscule to be of any importance?

I don't expect the extra kinetic energy to be a replacement for dark matter, and suspect the effect may be too small to be of much importance, but the kinetic energy distribution around a galaxy, intuitively on the surface, appears to have the right distribution, so I'm asking generically, do astrophysics factor this in their models, or not, and if not, should it?

Another way of seeing this is to calculate $$GM/Rc^2$$, a ratio which tells you the relative size of errors you will get in ignoring GR in any calculation.
For our Galaxy, we might say there is about $$M\sim 10^{11} M_{\odot}$$ within $$R\sim 15$$ kpc, and the ratio is then $$3\times 10^{-7}$$, which indicates Newtonian gravity is fine for most purposes.