Dark Matter(s)
We remind the reader once again that dark matter, whatever it is, does not couple strongly to
light. If it is composed of particles that were present in the early Universe (as is likely for at least
a significant fraction of it), those particles could begin to form structure before the decoupling of matter and
radiation permitted structure to survive for the normal matter.
As we have noted earlier, this may be key to why
structure could form so early in the young Universe. The
CMB fluctuations may represent the
visible matter response (growing but still tiny
at the time of decoupling) to larger density perturbations already present in the dark matter.