If neutrinos have mass but do not interact much with normal matter, why can't they be the dominant form of dark matter in the universe?
A) Structure formation would have started with large objects fragmenting into smaller objects.
B) Structure formation would have started with small objects merging to form larger objects.
C) Neutrinos would decay over time and disappear, causing galaxies to fall apart.
D) Neutrinos would not gravitationally lens background galaxies.
E) Neutrinos are charged particles.

Question

Quizplus · Accepted Answer

Neutrinos with mass move at speeds close to the speed of light and, therefore, become non-relativistic or "cold" after the universe has cooled down. Because they were still relativistic at the time of recombination, they did not participate in acoustic oscillations and could not have been the primary source of density perturbations that seeded the growth of large-scale structures through gravitational collapse. Therefore, if neutrinos were the dominant form of dark matter, structure formation would have started with large objects fragmenting into smaller objects, which is not consistent with observations.

If Neutrinos Have Mass but Do Not Interact Much with Normal