"The sharpened map of the galaxy also raises new questions, such as how gravity affects the spiral structure in galaxies. The spiral pattern in the Milky Way is like most other "grand design" spirals in that it is approximately logarithmic, or shaped like a cyclone. Scientists believe that the spiral structure is driven by the gravity of the disk. "But we are seeing spiral structure even where gravity seems to be too weak to drive it. Why? How?" asks Blitz."

I wish the article had addressed thoughts on the role of Dark Matter in this structure. Obviously the researchers are well aware of it, so I'm curious about its role especially in the problematic areas 'where there are no stars seen'.