The Universe is full of structures and gravitational instability (GI) is the key idea which can explain how these structures form. Ideal models of any astrophysical structure are often isotropic in nature, in the sense, they are uniform in their respective geometry. Under a given geometry, they might be considered dynamically stable, where their own weight is supported by internal forces. Such idealized systems will never form structures.
However, if a self-gravitating system, at the time of its inception, has some irregularities in the distribution of its matter, then the regions with more matter will exert more gravitational force, and will draw more matter from the surrounding regions. In other words, the small perturbations that were initially present, will grow in time, causing the overall system to collapse or fragment.
In many astrophysical systems such as molecular clouds and protoplanetary disks, the competition is mainly between gravity, which pulls the matter together, and internal forces driven by pressure, rotation, or thermal motion, which resist collapse. If the gravity dominates, the configuration becomes unstable.
See also