The critical density is the average density of matter in the universe today that would be needed exactly to halt, at some point in the future, the cosmic expansion. A universe that has precisely the critical density it is said to be flat or Euclidean. If the density of the universe is greater than the critical density, then not only will the expansion be stopped but there will be a collapse of the universe in the distant future. In this closed universe scenario, the universe will eventually implode under its own gravitational pull, leading to an event known as the Big Crunch. If the density is less than the critical density, an open universe scenario plays out in which the cosmic expansion will continue forever.

The critical density is calculated to be about (1 to 2) × 10^-26 kilograms per cubic meter– about 100 times greater than the average density inferred from all the known visible matter in the form of galaxies. However, when the inferred presence of dark matter and, possibly, of dark energy, is take into account, the universe appears to be pretty close to the density called for by the flat scenario.