Q&A: Dark Matter

Q:
I found a web site that explained Dark matter as the matter outside of a galaxy that we can't see. It said that it is believed to be there, because of the gravitational rotation of the galaxy. That doesn't make sense to me. The galaxy spins like a wheel, fast toward the center, and slower at the edge. But isn't it all spinning at the same speed? The outer part has a lot more "ground" to cover before it can make a full rotation, whereas the inner part of the galaxy has a shorter amount of ground to cover. Even with all of this, I don't understand how Dark Matter ties into it.

A:
The first indication of dark matter came in the 1930's when Fritz Zwicky looked carefully at a cluster of galaxies called the Coma Cluster. He was able to measure the velocities of some of the outermost galaxies. In order for those galaxies to be bound to the cluster, the gravitational binding energy had to be greater than their energy of motion (kinetic energy). There is a nice and simple calculation for finding the velocity needed to escape the gravitational pull of a body. When we want to shoot a rocket from Earth into space, it must be traveling fast enough so that Earth's gravity cannot pull it back down, which is the same as saying it must be traveling faster than Earth's escape velocity.

It turned out that the galaxies that Zwicky observed were going too fast to be gravitationally bound to the cluster! This was our first evidence that there might be a lot of matter that we cannot see holding this galaxy cluster together. Either that, or our theory of gravity needs to be modified at large scales (although this seems unlikely to me). Or, the observations were not interpreted the correct way.

But, other observations since then show a similar phenomena, that in order to hold many big groups of galaxies together, there must be a lot of matter that we just cannot see with any of our telescopes.

For more information, visit the X-ray Astronomy Field Guide on dark matter:
http://chandra.harvard.edu/xray_astro/dark_matter.html

and both optical and X-ray images of the Coma Cluster, along with a description of its importance:
http://chandra.harvard.edu/photo/2002/0150/