 The SM adds small departures from strict uniformity in order to account for the formation and evolution of structure. Due to gravitational instability, such perturbations are enhanced dynamically—the density contrast of an initial region that differs from the average density grows with time. Sufficiently small fluctuations can be treated as linear perturbations to a background cosmological model, governed by an evolution equation that follows from EFE. Yet as the fluctuations grow larger, linearized perturbation theory no longer applies. According to the SM, structure grows hierarchically with smaller length scales going non-linear first, and larger structures forming via later mergers. Models of evolution of structures at smaller length scales (e.g., the length scales of galaxies) include physics other than gravity, such as gas dynamics, to describe the collapsing clumps of matter. Cold dark matter (CDM) also plays a crucial role in the SM’s account of structure formation: it clumps first, providing scaffolding for clumping of baryonic matter.