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The Coma Cluster is what famously lead Fritz Zwicky to the conclusion that Dark Matter exists. As the velocities of the galaxies within the cluster were too fast for them to remain within orbit of each other if their masses were only baryonic.

So my questions are

  1. Have we seen this missing mass effect with (all) other clusters?
  2. From models, do we expect there to be missing mass outside of the galaxies themselves (inter galactic dark matter)?
  3. (Assuming #1 & #2 are yes) Is there a consistent measured ratio of the missing inter galactic mass vs the baryonic cluster mass?
  4. (Assuming #3 is yes) What is this ratio called and can anyone point to any papers?

If #2 is no, I assume this means adding the dark matter halos to just within the galaxies to fix their rotation curves also fixes for their cluster velocities? And the answer #3 in this case is then just the Tully Fisher ratio?

Massive thank you for your help!

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  1. Yes, all clusters, where sufficiently precise measurements available, require dark matter to explain their dynamics.

  2. Yes, the dark matter should be concentrated around galaxies (though not as much as luminous [stellar] matter) but there is expected to be dark matter sitting in the gravitational wells made by clusters.

  3. Not sure what you are asking here. It is hard to decide what "intergalactic" means. Note that the intra-cluster medium contains plenty of (hot) baryonic matter too. Generally speaking what you get from X-ray and lensing observations is a profile of dark matter within the cluster. This tends to follow the profile of luminous matter but there can be differences. Unlike normal matter, dark matter isn't stripped out of galaxies as they interact with the intra-cluster medium or indeed with other galaxies. The "Bullet cluster" is the poster child for the differing distributions of luminous, X-ray emitting and dark matter after an interaction between two clusters.

  4. The ratio is the almost universal ratio of 5:1 between dark and baryonic matter.

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    $\begingroup$ Thank you, so much this is really helpful. Is there a name to this ratio or any papers you'd recommend? $\endgroup$
    – Suzie Q.
    Commented Mar 15, 2022 at 11:04

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