The image above shows the Bullet Cluster. Also known as 1E 0657-56, this is a pair of clusters of galaxies some 3.4 billion light years away. As Jon Davies told us in yesterday’s Astrolunch meeting however: you should be careful about believing everything you see. This image is not a regular photograph by any means. You can see the galaxies scattered about with an orange glow. This much is familiar. Layered onto this optical data is the pink, X-Ray picture from the Chandra telescope. This pink light is actually the high energy X-Ray radiation from a hot gas that permeates the cluster. Again though it is not unusual to see two different wavelength regimes seen in the same photograph.

What is unusual is that the blue ‘light’ seen here is not photographic in nature at all. It is the location of the mass in this region deduced by weak gravitational lensing - it is not a real effect but rather a mathematical interpretation of where the mass should be. As you can see it does not line up with the visual traces of the mass that we see as light and X-Ray material.

This is because it is believed that the Bullet Cluster shows us quite nicely where the dark matter can be found in this cluster. The interpretation of this image by the researchers who have studied it is that the hot, pink gas is the energy released by ordinary matter in this pair of clusters as they have collided with one another. The energy of the collision has excited the gas to emit in the X-Ray.

However not all the mass in the region is ordinary (something believed to be true about most of the universe). Dark matter is material that has mas but doesn’t interact with other, ordinary matter by the usual routes. It does not feel magnetism, or electrical forces or emit light. But is stays tethered to the ordinary matter by gravity alone.

As these two clusters collided the dark matter passed through like an ordinary pair of gaseous objects would, where as the regular, every day material became heated and disturbed and distorted in shape.

There is a wonderful video of a simulation of this hypothetical collision which you can find here.

This lovely picture adds to the already heated debate among the astrophysical community as to the existence of dark matter.