Technically Speaking: Coupling of Radiation and Matter

Until the time of hydrogen atom recombination in the early Universe, radiation and normal matter were strongly coupled to each other through electromagnetic interactions (and by the weak interactions, but only for the first second after the big bang). Since the anisotropy in the cosmic microwave background is of order 1 part in 100,000, this implies that at the time of decoupling of normal matter and radiation the normal matter was smooth on the same scale. Thus, no significant density fluctuations could grow in normal matter until decoupling.

But dark matter, by definition, does not couple strongly to light. Thus, it could have decoupled much earlier from the photons and begun to clump gravitationally. These clumps could have provided the seeds for early galaxy formation as soon as the normal matter decoupled from the photons and could begin to condense.

Role of Dark Matter

Dark matter in the form of elementary particles appears to have been essential to the formation of structure in the early Universe.
Visible Matter Won't Do
Observational evidence at high redshifts suggesting that galaxy formation was already well underway by the time the Universe was 10 percent of its present age is almost impossible to understand based on the visible matter of the Universe. There simply isn't enough visible matter to have clumped gravitationally on a short enough timescale to initiate such early galaxy formation.

Initial Seeds
However, because dark matter does not couple strongly to photons, it could begin to clump together earlier than the normal matter (see the adjacent box), and because there is so much more of it, it could clump more effectively. Thus, it is likely that dark matter provided the initial regions of higher than average density that seeded the early formation of structure in the Universe. This animation illustrates the role of dark matter in forming the initial structure in the Universe.