So either somehow that explanation doesn’t apply there (but why?) or dark matter is real, and just somehow absent from some galaxies.
Link: https://www.space.com/19-galaxies-missing-dark-matter.html
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.
Yes, you heard that correctly, the Bullet cluster is a PROBLEM for dark matter, not evidence for it.Is the maximal speed of the slingshotted particle something like twice the speed of the big objects? Do they escape or they just form a cloud that gets hotter?
For this subject, it's important that the simulation conserves energy. Most naive numeric simulations don't do that, and even symplectic simulations increase/decrease the total energy slowly.
They escape.
yeah, having done this before, I am 100% sure that the simulation will be inexact due to numerics, and that the inexactness will be worse around these phenomena, but we are probably talking, gut feeling say, under 20% in most scenarios where you see an escape (I think the observable error is effectively unbounded because you could in theory get two particles within one ULP or even two particles that collide and cause a NaN error)... But in general I don't think that changes the qualitative nature of the phenomenon. There's probably a reasonably easily derived "starting from three particles at rest" where you can see one of them escape from the other two; if not 3 then four.
The best attempt I've seen to summarize this is in this paper https://arxiv.org/abs/1412.7719 which argues that there's roughly a 10% chance of getting something as extreme as the Bullet Cluster under Lambda-CDM. So, mildly unlikely, but not even approaching the (problematic) P < 0.05 threshold that's emblematic of the "replication crisis" in fields like experimental psychology.