How much faster are the stars on the outskirts of galaxies going than they are 'meant' to as predicted by general relativity? this is in reference to dark matter. I've look on the internet for this but I can't find precisely what I am looking for.
 A: This question can be answered by looking at a galaxy rotation curve such as this one for the spiral galaxy M33, taken from the linked Wikipedia article:

From the plot, the stars at the edge of M33 are moving maybe 75-100 km/s faster than you'd expect them to based on the mass in the visible disk.
As noted by Qmechanic, this prediction can be comfortably made from Newtonian gravity, as the speeds and average gravitational fields involved are not so strong as to necessitate the use of the full machinery of general relativity.
A: It depends on the specific galaxy, but mostly you'll find a "Flat rotation curve" as the one shown in the other answer.
In the dark matter paradigm, there's a dark matter distribution around the galaxy which varies from one type of galaxy to another, and sometimes within the same type of galaxy (you may want to research the following topics: Cusp-core problem, NFW profile). This distribution is affected by lots of different processes in the galaxy (baryonic feedback, for example), and the rotation curve depends on this distribution.
In other paradigms such as modified gravity theories it gets more complicated. MOND is a fairly simple one and can predict these rotation curves pretty accurately. You can probably derive a "how much faster" relationship using MOND.
