Im not sure I understand exactly why the picture/measurements of the bullet cluster are evidence for dark matter?
This image of the Bullet Cluster shows a clear separation between regions of hot gas seen in x-rays (pinkish color) and regions of mass inferred from gravitational lensing (blue-violet color). This cluster is actually two clusters that have collided and passed through each other: the hot gas region in between is due to intra-cluster gas clouds which have collided with each other, slowed down, and heated up (you can even see a shock front). This is exactly what one would expect gas made of normal matter to do. But the dark matter associated with the galaxy clusters has just kept on going, gravitationally bound to the clusters. This shows that there exists massive matter that does not interact with normal matter except by gravity. If the dark matter were normal matter, it wouldn’t be separated from the hot gas.
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