The Chandra orbiting X-ray Observatory has come through once again by detecting the results of a galactic cluster-scale collision that has forced the visible matter apart from it's normally associated halo of dark matter to such an extent that the gravitational effects of each are readily apparent.
Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. The discovery, using NASA's Chandra X-ray Observatory and other telescopes, gives direct evidence for the existence of dark matter.
Before this event, we have had a pretty good handle on where the visible parts of the universe were, but that only accounts for about 4% of the total energy in the universe, and the hunt for the remainder has been pretty fierce while all sorts of open questions around what is dark energy or matter persist.
4-Panel Illustration of Cluster Collision
These stills show four stages from an artist's representation of the huge collision that is taking place in the bullet cluster. Hot gas, containing most of the normal matter in the cluster, is shown in red and dark matter is shown in blue. During the collision the hot gas in each cluster is slowed and distorted by a drag force, similar to air resistance. A bullet-shaped cloud of gas forms in one of the clusters. In contrast, the dark matter is not slowed by the impact because it does not interact directly with itself or the gas except through gravity. Therefore, the dark matter clumps from the two clusters move ahead of the hot gas, producing the separation of the dark and normal matter seen in the image. View Animation
Here is the report from the official Chandra Chronicles Newsletter describing the ground-breaking observation.
Here is a great post from Cosmic Variance with more detail.