I've recently come across some doubters as to the existence of dark matter. If you find yourself among that group, please read the following article on New Scientist and my comments. Then you can jump on the DM bandwagon with me! Dark matter and normal matter 'divorce' in cosmic clash: "Dark matter seems to have separated from normal matter in a mammoth collision between two galaxy clusters. The results bolster observations of a similar collision called the Bullet Cluster and put rough upper limits on how strongly dark matter interacts with other matter and itself. But so far they cannot rule out any of the leading dark matter candidates... ...Now, isolated clouds of dark matter have been observed in a collision between two massive clusters of galaxies lying 5.7 billion light years away." Check out the story here. There's a few important details the New Scientist article left out: First some background. This is the second time that we have seen two clusters collide, the first time was seen in the Bullet Cluster. In both cases, after the clusters collided the majority of normal matter is stripped from the clusters, and we see a separation between normal matter and dark matter. Now for the details. You probably saying to yourself, "stars collide and get swept up when clusters collide?, That's not right." And you'd be correct. When galaxies or clusters collide stars don't hit each other. The key to understanding the Bullet Cluster results is that most (~90%) of normal matter in a cluster resides in hot intergalactic gas. It's this gas that "hits" the gas in the other cluster. So the gas (i.e. 90% of normal matter) in the two clusters run smack into each other while the cluster's galaxies and stars and dark matter just sail on through leaving their hot gas far behind. Finally, using some techniques (weak lensing and maybe strong lensing too) based on the fact that light is bent by mass, we find the mass distribution (only the 2-D projection onto the sky) of this system. Then we see that sure enough, the dark matter just sailed on through with the galaxies and stars! These two results (this new cluster collision and the bullet cluster collision) are not compatible with MOND. MOND is an attempt to modify gravity so we don't need to postulate the existence of dark matter. However, MOND cannot explain these cluster collisions! In order to explain these cluster results, MOND (or some other gravitational theory) would need to say that the gravitational force vector does not point toward the mass creating that force! That is, in this new theory, I wouldn't be falling towards the center of the earth, but in some other direction! Highly unlikely.* Hooray for Dark Matter! If you're still not convinced or can't understand my explanation, then check out this excellent blog post by Sean Carroll at Caltech (it was written only about the Bullet Cluster). *Please ignore the pseudo-forces due to Earth not being an inertial frame of reference. Imagine a non-rotating earth that isn't orbiting anything.