voids

Voids are empty spaces within a material, structure, or system. They occur when matter is absent from a region that would otherwise be filled, and can range in size from microscopic pores to cosmic-scale cavities.

In cosmology, cosmic voids are vast underdense regions in the large-scale structure of the universe. They contain few galaxies and a low matter density compared with the cosmic mean. Typical voids span tens to hundreds of megaparsecs and are bordered by a network of filaments and walls. Their existence and growth help test models of gravity, dark energy, and the evolution of the early universe.

In materials science and engineering, voids refer to cavities within solids that arise during solidification, casting, welding, or sintering, or from gas entrapment and shrinkage. Voids reduce strength and stiffness, increase permeability, and can concentrate stress to initiate cracks. Detection methods include optical and electron microscopy, X-ray computed tomography, and ultrasonics. Mitigation involves processing controls, degassing, venting, and optimized cooling or solidification.

In geology and mining, voids are cavities within rock or earth, such as caves, karst voids, or mined-out spaces. They influence fluid flow, rock stability, and geotechnical risk. Assessment uses geophysical surveys, boreholes, and monitoring instruments; hazards include ground subsidence and water inflow.