IIRC, this concept is where the idea of dark matter came from. Given the mass of a galaxy, they have more angular kinetic energy (or rather they spin faster) than they should given detectable mass alone. Gotta be something making galaxies spin faster than they should, and that something is what was labeled dark matter.

Dark matter is just a placeholder until we find whatever that "something" is, or a better model of why this is happening arises. All of that is predicated on the estimate of kinetic energy you inquired about

Well, the mass of the Milky Way is very roughly 2 × 10^42 kg, and most of the matter is orbiting at 200,000 meters per second. My napkin says ~ 4e52 joules.