Curvity, the Bullet Cluster and MOND
It is perhaps the wikipedia article on the Bullet Cluster that best spells out it's importance:
In theories without dark matter, such as Modified Newtonian Dynamics, the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. However, the lensing is strongest in two separated regions near the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of collisionless dark matter.
To clarify further, gravitational lensing [wikipedia.org] occurs when a massive object, be it a sun or a galaxy, bends the light from other objects behind them. A basic assumption of MOND is that the acceleration modification, i.e. the bending of light, occurs within the baryonic matter not outside of it. Remember that MOND was built to explain the flat rotation curves in the outer rim of galaxies and fails with the Bullet Cluster because lensing shows an "acceleration" outside the baryonic matter.
The primary postulate of Curvity is that baryonic matter creates both positive gravity within it self, which we experience daily, and depending on mass and distance, creates a ring of negative gravity around itself. The negative gravity could create exactly the type of lensing seen by the Bullet Cluster and would be separate from the baryonic matter! Curvity's negative gravity would also be "collisionless", just as regular positive gravity is.
The Bullet Cluster can also be used to separate MOND from Curvity. The lensing (blue) outside the gas (red) in the Bullet Cluster is an incredible real-time picture of Curvity at work, which MOND cannot explain completely.
The next question is which direction does negative gravity lens the light from an object behind it? A logical guess would be in the opposite direction of positive gravity, but is it possible to tell the difference? Not likely because it is not possible to know the original direction of the objects being lensed, which means it isn't possible to tell if the refraction is positive or negative. All that is possible is to tell that refraction has occurred.
The other possibility that Dark Matter could be a particle is also being ruled out. For example, most particles decay at some point through out their life time.
Riemer-Sorensen and her team searched for the X-ray hallmark of decaying dark matter in the Bullet Cluster. The formation is one of only a handful thought to show clear evidence of dark matter, which makes up about 85 percent of all matter in the cosmos.
The astrophysicists compared X-ray emission from one of the cluster's dark matter blobs to a desolate region nearby, but hardly any difference showed up.
"We didn't see anything," she said, explaining that the dark matter blob had about the same amount of X-rays that the "control" region without any dark matter did. 
Which means two things, the particles that make up Dark Matter don't exist or they take a very long time, like protons, [wikipedia.org] to decay.
- "Invisible Matter Won't Disappear Anytime Soon", Space.com, October 8th, 2007