How does the bubble cluster indicate DM again?
Do you mean the Bullet Cluster? The idea here is that there’s a separation between regions of normal matter (hot gas measured by x-rays) and mass concentration (measured by gravitational lensing). 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.
