密度差（Density contrast）是一种解释是星系和大尺度结构形成的理论，其方程序为:

一般认为，宇宙是从一团炽热微小的火球膨胀而成的，这就叫做大霹雳，而理论认为宇宙刚诞生时密度并非完全均匀的，其中有密度比较大和密度比较小的地方，而随这宇宙的膨胀和冷却，这些密度比较大的地方 会因为重力比较大而自行塌缩，这样就会行成星系和大尺度结构。

The Density Contrast is a parameter used in galaxy formation to indicate where there are local enhancements in matter density.

It is believed that after inflation, although the universe was mostly uniform, some regions were slightly denser than others with contrast densities on the order of 1 trillionth. As the horizon distance expanded, the enclosed causally connected (i.e. gravitationally connected) masses increased until they reached the Jeans mass and began to collapse, which allowed galaxies, galaxy clusters, superclusters, and filaments to form.