[Cboldt]

"dark matter" is used to explain the measured rotation of galaxies. If the conventional math is used, the stuff closer to the center of a galaxy would have a shorter "orbit time" around the center of the galaxy, like Mercury has a shorter year compared with earth, pluto, and other planets. But galaxies appear to rotate as if the bulk of stars have the same "orbit" time about the center of the galaxy, regardless of how far they are from the center of the galaxy.

[Do the math]

Yes, but again it is a problem with faulty assumptions.