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According theories that work best for the structure formation which happened first in the history of the evolution of the Universe, the decoupling of dark matter or the decoupling of photons? How sure are we about it?

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  • $\begingroup$ why do you think that there has been a decoupling of dark matter? Decoupling means "very few interactions" but the basic interaction of dark matter is gravitational and it is still going on. $\endgroup$
    – anna v
    Apr 7, 2019 at 19:09
  • $\begingroup$ In the standard thermal DM scenario, in the early universe it was in equilibrium with the SM particles in the thermal plasma. Then, the number density per comoving volume stopped decreasing i.e. annihilations stopped and it got frozen out. After that kinetic decoupling occured at which the scattering of DM with the SM particles stopped. Since then it has been free streaming. @anna $\endgroup$
    – SRS
    Apr 7, 2019 at 19:43

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This is the Big Bang time line:

histun

In this review the freeze out of dark matter candidates happens in the neucleosynthesis time, above shown at less than a second. As the photons decouple at 380.000 years there can be no doubt that they come last. It would need a drastically different model to doubt the sequence.

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