If you're going to name a cluster of galaxies after the mythological character who unleashed evil on the world, this particular is a pretty good choice. It's a cacophonous, mixed-up mishmash of four older clusters, plus plenty of dark matter.
Officially, this is cluster Abell 2744, and it's about two million light-years across. Because the cluster formed from the collision of four earlier clusters over the last few billion years, the cluster is not evenly distributed, giving it a strange, chaotic appearance. While the jumbled galaxies take up plenty of space in this image, there's also lots of X-ray emitting hot gas, which is represented by the vast red region.
But what about the dark matter? Obviously, we can't see that directly, but we can infer its existence due to gravitational lensing. This refers to how much the light from still more distant objects beyond Pandora's Cluster is bent by the mass in the cluster. To account for the high degree of gravitational lensing, about 75% of the total mass of Pandora's Cluster must be composed of dark matter, and that's colored blue in the image.
Amazing, Pandora's Cluster doesn't even have the worst reputation of the Abell clusters, a group of about 4,000 galaxy clusters cataloged by George Abell in 1958. That honor must surely go to Abell 520, which has the wonderfully unflattering name of The Train Wreck Cluster.