Milky Way's Spiral Arms Are the Product of an Intergalactic Collision Course; Models Show Dark Matter Packs a Punch

A dwarf galaxy called Sagittarius has collided twice with our Milky Way Galaxy, according to telescope data and detailed simulations, and is lined up to run into us again. When these galaxies collide, it sends stars shooting out in long loops which eventually build up to form a familiar ringed arm. Each time this collision has occured though, Sagittarius has been torn to pieces by about 80 to 90 percent and loses much of it's dark matter to form new arms for the milky way and its greater gravity. Although this may seem alarming, we do not have to worry at the moment, as scientists predict that the next collision will occur in about 10 million years.

http://www.sciencedaily.com/releases/2011/09/110914131338.htm