Dark Matter

New York Times Across the Universe Blog, Searching for the Dark Side:

[Excerpt]

We have long known that there is more to our universe than we can see. The great eccentric of American astronomy, Fritz Zwicky, realised in 1930 that the galaxies within galaxy clusters were moving more rapidly than they should be. The only explanation was to assume that there was more matter present than scientists had thought, so that the increased gravitational attraction could prevent the galaxies from escaping. A similar problem exists on the scale of individual galaxies; if only the visible disk of a galaxy such as the Milky Way existed, the galaxy would fly apart in just a few million years. The solution is the same; if the Milky Way is embedded in a halo of dark matter, then all is well.

What is this dark matter? We know very little about it; whatever it is, it interacts via its gravitational attraction with normal matter, but not with light. For a while the best candidates were objects such as black holes, or free-floating planets in the Milky Way halo, but searches for the effect that such objects would be expected to have on background stars showed that very few exist. Instead, most cosmologists now believe that dark matter is composed of slow-moving exotic particles that have yet to be identified.

The current battleground is known, appropriately enough, as the Bullet Cluster of galaxies. The picture above needs a little explanation; the pink is emission from hot gas within the cluster, seen in X-rays. The blue region represents the underlying mass distribution, as determined by studying the distortion in the shape of the background galaxies as light passes through the Bullet Cluster in the foreground.

The Bullet Cluster is actually two clusters of galaxies in collision. As they hit, the gas contained within each interacts and heats up, leading to the X-ray emission we observe, and the gas slowing down. The dark matter, however, interacts only via gravity – a much weaker force – and so overshoots the center. This cluster is important because it allows us to actually see the difference in behavior between ordinary and dark matter.