"Astronomers have come to the humbling conclusion that most of the matter in the universe, approximately 80 percent, is in the form of dark matter. Humbling because they do not know what it is. The two known types of dark matter, massive neutrinos and black holes, are thought to be a minor portion of the overall dark matter budget."
"These results are so surprising that some astronomers who accept and even contribute to this work have called the universe "preposterous" or "extravagant." In the opposite camp is a small, but passionate, group of astronomers who think there are serious flaws in the currently accepted cosmology." Therefore, they suggested changes to the theory of gravity, i.e. Modified Newtonian Dynamics (MOND), which make it stronger than predicated by Newton and Einstein on intergalactic scales.
Personally, I would like to see the dark matter theory to win, because of my belief that the foundamental theory that governs our universe should be concise and elegent, instead of piece wise function like. But we never know, just like general relativity replaced Newton's theory, and quantum mechanics complementing general relativity.
One way to decide between these two competing theories, dark matter vs. MOND, is to find a system where normal matter is seperated from dark matter. If dark matter does exist, we should be able to see the gravitational field distorted solely by the dark matter. The observed collision provided just such an opportunity. In a collision between two clusters, normal matter exerts a drag force, similar to air resistence, thus slows down itself. In contrast dark matter should not slow down because it doesn't interact directly with itself or the normal matter except through gravity.
For a detailed explaination, see Cosmic Variance.