@7, 9: Here's the NASA pictures of the Bullet Cluster. The Bullet Cluster is actually two separate clusters of galaxies that collided "recently" (i.e. only a few million years ago). The picture is actually highly important because it's direct visual proof of the existence of dark matter.

Despite all the stars in each individual galaxy, the vast majority of a cluster's mass (if you look at just the "normal", "baryonic" matter) comes from the vast, thin clouds of wispy neutral hydrogen gas floating between the galaxies. When these two clusters collided, the galaxies (yellow/white in the picture) passed straight through because a galaxy is much denser than a hydrogen cloud. Meanwhile, the two neutral hydrogen clouds (red in the picture) smacked into each other, heating up enough to ionize and produce X-rays (which the Chandra X-ray telescope can see).

Funny thing is, though, that General Relativity lets you weigh an object by looking at the things behind the object. The light passing through is distorted because gravity bends the light beams. This "gravitational lensing" causes funny optical effects that you never see in actual galaxies, like rings and curves with sharp edges, so it's easy to detect with a computer. So they measured the lensing with optical telescopes, then artificially shaded the picture (in blue) to show where the gravitational lensing was strongest -- and therefore where the most mass is.

Since most of the "normal" matter from the cluster was in the gas clouds (red) that smashed together in the collision, we would expect that to be the location where the most gravitational lensing is (blue). Instead, the maximum gravitational lensing is not where the maximum concentration of hydrogen is. The blue and red are separate. The bulk of the mass passed straight through... even though the galaxies themselves don't weigh enough to account for it.

The picture is a very elegant proof that dark matter physically exists, and that it doesn't interact strongly with normal matter. Which is pretty amazing, when you consider that dark matter was originally proposed to explain why the outer edges of galaxies rotated too quickly.