Before the present general relativistic cosmological model was developed, Albert Einstein proposed a way to dynamically stabilize a cosmological scenario that would necessarily collapse in on itself due to the gravitational attraction of the matter constituents in the universe. Such a universe would need a source of "anti-gravity" to balance out the mutual attraction, a scalar term in Einstein's equations that would come to be known as the cosmological constant.
Einstein's first attempt at modeling relied on a cosmological constant that was finely tuned to exactly balance out matter curvature and provide a framework for an infinite and unchanging spacetime metric in which the objects of the universe were embedded. This happens to be the same as a special case of the current cosmological model where the cosmic scale factor is unchanging and the density seen in the Friedmann equations is equally divided between the cosmological constant and matter.
Willem de Sitter would later generalize Einstein's scalar potential model to a universe model that would expand exponentially. As the early development of the Big Bang theory began, De Sitter would be falsely credited for inventing the expanding universe metric because of this. In reality, it was the work of Alexander Friedman and Georges Lemaître who established the metric that would come to be the most accepted for cosmology. Nevertheless, De Sitter's model appears in two places today: in the discussion of cosmic inflation and in the discussion of dark energy dominated universes.