Sean Carroll hits a home run in explaining some recent science about showing the existence of Dark Matter. I initially had used the word prove, but in science you really never prove anything; you instead add evidence in favor of a theory. Some theories, like evolution, have so many compelling experiments and observations crossing discipline boundaries that they become compelling to the point of proof.
I urge your to read the article above. This is probably the most interesting experimental results in the last 20 years. Wow. From the article:
astrophysicists led by Doug Clowe (Arizona) and Maxim Markevitch (CfA) have now compared images of the gas obtained by the Chandra X-ray telescope to “maps” of the gravitational field deduced from weak lensing observations. Their paper is available here in pdf, and will appear on astro-ph this evening. And the answer is: there’s definitely dark matter there!
Congratulations, Doug and Maxim! Thanks, NASA! What a great job!