What you see is the collision of two galaxies over billions of years, albeit virtually.

As physicists at CERN investigate the smallest particles in the universe, US scientists are studying the behavior of the largest cosmic structures in existence. A team at the University of Chicago Flash Center and the Harvard-Smithsonian Center for Astrophysics used an Argonne National Laboratory Supercomputer to identify elusive dark matter. The researchers simulated the motion and collision of galactic clusters — some of the largest structures in the universe — to infer dark matter’s influence, as it cannot be observed directly. Dark matter greatly influences gas and galaxies over trillions of light years.
 
Furthermore, these collisions more accurately predict the interaction of both normal and dark matter (it is thought dark matter constitutes 23% of the universe’s observable mass, while normal matter only makes up 4.6%). The model initially simulates clumps of dark matter particles as they coalesce via an oxymoronic process called 'violent-relaxation.' Subsequent presentations portray the ‘gaseous-galactic clusters’ interacting directly.

Finally, the dark matter particles and gases are overlaid onto one another and their intricate interactions are played out. The dark matter cores of each galaxy collide and move past each other. Once settled, their motion enables the normal gaseous matter to interact and mix. See the hypnotizing cosmic dance for yourself by clicking on the image above and playing the video. (Watch it at 720p for highest quality.)

- Adrian Giordani, iSGTW