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A cosmic void is a large region with lower matter or galaxy density than its surroundings in the large-scale structure of the universe. It is not an exactly empty volume; voids can contain galaxies, gas and dark matter.

Voids emerge as initially underdense regions evolve relative to denser filaments and walls. An observational void depends on the tracer population, survey selection and identification algorithm, so a galaxy-defined void need not coincide exactly with a matter-defined underdensity. Their shapes, abundance and dynamics can test cosmological structure-growth models.

The cosmic web is the connected pattern of matter sheets, filaments and dense nodes surrounding cosmic voids. Collapse is generally anisotropic: a region can contract strongly in one direction before the others. The Zeldovich approximation makes this mechanism explicit through the principal directions of the initial deformation. Collapse along one, two or three directions gives the intuitive sequence of sheet, filament and compact concentration, although subsequent shell crossing and nonlinear dynamics complicate that picture.

The large-scale structure of the universe is the spatial organization of matter into dark matter halos, galaxy clusters, sheets, filaments and cosmic voids. Galaxy clustering traces this pattern, while the underlying matter also includes material that does not shine. Gravity amplifies initial linear cosmological density perturbations, but expansion and the matter's dynamics determine which scales grow efficiently. Comparing observed statistics with models of cosmological structure formation turns this pattern into a test of the initial conditions and subsequent evolution.