Abell 2744 is a cluster of galaxies; thousands of galaxies swarm inside of it. But new observations reveal a startling result: it's actually the collision of four separate clusters of galaxies, all slamming into each other at the same time! The amount of material involved is staggering, beyond belief: over four hundred trillion times the mass of our Sun! That's well over a thousand times the mass of our entire Milky Way Galaxy.
This picture shows the object, nicknamed the Pandora Cluster. It combines visible light images from Hubble and the Very Large telescope (shown in blue, green, and red) - which show gas and stars - with X-ray images from Chandra (shown in pink) which picks out extremely hot gas in between the galaxies, heated by the collision. The region colored vivid blue is actually a model showing where dark matter lies: this invisible stuff makes up most of the budget of the Universe's mass, but is not directly viewable. However, it has gravity, and that distorts the light coming from more distant galaxies behind it. By carefully mapping out that distortion, the location and amount of dark matter in the cluster can be determined. Clusters like Pandora house a lot of dark matter, and are great places to look for it.
Dark matter doesn't interact with normal matter, but regular old gas sure does. When clusters collide, so does their gas, which can screech to a halt in a head-on collision, while the dark matter blows right on through. By mapping where normal matter is compared to where the dark matter is, the history of the cluster can be found, and that's how astronomers figured out four separate clusters were involved.
The next time you get involved in a fender bender in a car, keep that in mind! Things could've been a lot, lot worse.