While astronomers continue an emotional debate over the definition of a planet, real science goes on. One of my friends, Doug Clowe, has headed an investigation into the dark matter content of a cluster of galaxies. What Doug and his collaborators found was that, in one cluster, the dark matter doesn't line up with the visible matter. This is solid evidence that dark matter is some strange, exotic thing and not just ordinary matter or a misunderstanding of our theory of gravity.
In the picture above, there are really three bits of information. The white and yellow are pictures of galaxies taken with an ordinary telescope. The pink "glow" is extremely hot gas, so hot it glows only in X-rays. And the blue is a map of the gravitational force, made by careful analysis of how that gravity bends and distorts light from more distant galaxies.
The amount of gas glowing in the X-rays is quite ordinary matter -- hydrogen, helium, iron, carbon, etc., -- heated to 10 million degrees. In fact, the amount of matter in the pink glow is much more than the amount of matter in the visible galaxies. So the pink glow really traces out the "ordinary" matter in the picture. The pull of gravity, which comes from a combination of dark matter and "ordinary" matter, will be strongest where there is the most matter. So, since the blue and pink do not line up, the matter that causes the gravity is NOT the "ordinary" matter that causes the X-rays. So it must be dark matter! This marks a new, independent confirmation of dark matter.
What is this dark matter? Astronomers and physicists have some ideas, but we have never directly detected dark matter in a laboratory. Such a detection would prove beyond any doubt that dark matter exists (and would win the team a Nobel prize). But, for now, astronomers can rest comfortably that dark matter actually exists.