The rotation speed of stars around galaxy

The only evidence they have for the existence of Dark Matter (Neil DeGrasse Tyson hedges his bets on this and refers to it as 'Dark Gravity') seems to be this, which I'm sure you've all seen dozens of times:



Thanks to Dinero, who reminded me a while ago about Shell Theorem, which makes doing the calculations a lot easier. I assume that the same logic applies to disk galaxies as it does to spheres, ...