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http://backreaction.blogspot.com/2021/05/dark-matter-situation-has-changed.html http://backreaction.blogspot.com/2021/05/dark-matter-situati... > Then there ar

http://backreaction.blogspot.com/2021/05/dark-matter-situation-has-changed.html http://backreaction.blogspot.com/2021/05/dark-matter-situati... > Then there are collisions of galaxy clusters at high velocity, like the bullet cluster or the el gordo cluster. These are difficult to explain with particle dark matter, because dark matter creates friction and that makes such high relative velocities incredibly unlikely. Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.

halter73 4y ago
That's funny. The parent's NASA link uses the high relative velocities as "direct evidence for dark matter and supports the view that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity." The lack of friction seems obvious with this view of limited interaction between dark matter particles.
- MrPatan 4y ago
  That's also how I understood it. No photons, no electrical interaction, no friction.
  - floxy 4y ago
    Chandrasekhar dynamical friction is just a function of gravity: https://en.wikipedia.org/wiki/Dynamical_friction https://en.wikipedia.org/wiki/Dynamical_friction
    - MrPatan 4y ago
      Well, ok, but that means there's still _less_ friction with those particles because they don't interact electrically, right? So I still would expect the same result of the 4 blobs in different positions in space. What's wrong with that view?