When Galaxies Collide

Here we see Apr 148, which is two galaxies that have collided with one another creating a ring shape with a long tailed companion. This collided galaxy is in Ursa Major and is roughly 500 million light years away. When they collided it created a shock wave effect that first drew matter into the center and then caused it to expand outward in a ring.
When galaxies collide they create what astronomers call a perfect cosmic storm that can have enough energy output that is second only to the big bang and resembles that of a cosmic hurricane. Recently using space based x-rays astronomers have been able to see the products of galaxy collisions that before were only done in computer simulations. These observations have lead to the idea that the universe formed in a "bottom up" hierarchical structure where stars and galaxies collected and combined together to form larger galaxies and galaxy clusters. The Milky Way is actually believed to be part of a galactic club called the local group which could collide with the Virgo Cluster in a few billion years unless it is prevented due to the expansion of the universe.