= Cosmic void
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.
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