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>mostly made of dark matter (80-90%) and most of the normal baryonic matter is in the form of a hot plasma

if you can answer it in the short post, how it is established that it is dark matter and not cold gas?

There's lots of good evidence for something like dark matter in clusters. The nicest one is the Bullet cluster (lots online about this), where we can see the hot gas and mass are offset.

We can measure the shape of the dark matter in the cluster by various techniques (eg gravitational lensing). Cold material couldn't exist in a distributed form in the very hot cluster atmosphere very long. Also how could it get there intact? Also it's difficult to hide normal matter from being seen, particularly in this hot energetic environment.

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