As heavy as dark matter may be in today’s universe, galaxies would not have been able to form quickly without it. Galaxies formed from density fluctuations in the early Universe, and some had formed as early as 400-500 million years after the Big Bang. Observation with Wilkinson Microwave Anisotropy Probe (WMAP), Planck & other experiments give us information about the size of these density fluctuations. It turns out that the observed density variations are too small to have formed galaxies so shortly after the Big Bang. In the hot, early universe, energetic photons collided with hydrogen. and helium, and it kept them moving so fast that gravity wasn’t strong enough to combine the atoms together in galaxies. How can we reconcile this with the fact that galaxies have formed and are all around us?
Our instruments that measure the cosmic microwave background (CMB) only give us information about density fluctuations for ordinary matter that interacts with radiation. Dark matter, as the name suggests, does not interact with photons’. Dark matter could have had much greater density variations and could have merged into gravitational “traps” that could have started to attract ordinary matter immediately after the universe became transparent. As ordinary matter became more and more concentrated, it could have quickly turned into galaxies thanks to these dark matter traps.
As an analogy, imagine a boulevard with traffic lights every 800 meters. Suppose you are part of a large number of cars that are escorted by the police who are driving you. behind every light, even if it’s red. Even when the early universe was opaque, radiation interacted with ordinary matter, giving it energy, and carrying it away, sweeping past concentrations of dark matter. Let us now assume that the police leave the caravan, which then finds red lights. as traffic traps; Approaching cars now have to stop and are therefore grouped together. Similarly, after the early universe became transparent, ordinary matter only occasionally interacted with radiation and could therefore fall into the traps of dark matter.