Multiple observations show that visible matter alone cannot explain gravitational dynamics in galaxies and clusters, the growth of large-scale structure, or the detailed pattern of CMB anisotropies. Dark matter is a pressureless, gravitationally clustering component that helps perturbations grow before and after recombination.

In the matter-dominated era, small density contrasts $\delta = \delta\rho/\rho$ grow approximately as $\delta \propto a$ in linear theory. This growth is slower during radiation domination and altered again once dark energy becomes important. Understanding these regimes is essential for linking initial conditions to today's cosmic web.