While visiting the awesome APOD site Sunday morning, the question arose, "Why do so many galaxies have the 'spiral' shape?" The Oracle Google was consulted - here, here, and here. I still don't know why so many galaxies have the "spiral" shape.
"These arms are in fact density waves passing through the galaxy, with stars moving in and out of the waves. The arms themselves aren’t permanent structures made of the same clumps of stars." (1)
Oh. After five minutes of Intertunnel research, one comes to the conclusion that the scientists don't really know either. I find great comfort in that notion.