Actually, the flatness that Rubin discovered only applies to the outer bands. The inner and middle bands vary quite a bit in velocity. Some outer bands are nearly flat and some trend up as pictured above. Essentially, she discovered one of the important effects of dark matter.
But I wonder if this new finding is going to tell them anything about dark matter that they didn't know before? I mean, it seems like it's a pretty significant discovery... The outer regions of every disk galaxy rotating once every billion years, regardless of mass, etc. Really bizarre.