Two galaxies colliding and merging.
Generated using the simulation code GADGET-2 running on a small number of nodes on the HPC cluster at the Center for Computation & Visualization and rendered using IFrIT. The T= shows simulation time in billions of years.
The source code computes the gravitational forces between the ordinary matter within the galaxies and the dark matter halos surrounding them. The dark matter is not rendered in this visualization. The code was created in 2000 and last updated in 2005. It is optimized for massively parallel computers with distributed memory.
A somewhat better quality version can be viewed here.