By: Metacafe Affiliate U
Spiral galaxies are believed to be surrounded by hundreds of satellites made of dark matter. Some of these are visible but most are definitely invisible. Nevertheless, they still contain mass and gravity that causes the disk to vibrate like a drumhead as they pass through it. These vibrations grow into the spiral structure that we see in pictures of galaxies. In this animation, we fly around a galaxy over a timespan of 10 billion years and watch it respond to satellite bombardment creating recurring spiral arms.
Animation by John Dubinski
Bach performance by Lara St. John
http://www.magnatune.com/artists/lara_st_john (Creative Commons License)