Astronomers use the Greek capital letter omega (Ω) to represent the concentration, or density, of matter in the universe. Sometimes, a subscript M is added (Ω_M) to make clear that this is the concentration of matter; at other times, two subscripts are used to distinguish the concentration of dark matter (Ω_DM) and that of baryonic or non-dark matter (Ω_B).

If dark energy does not exist, then the matter density in the universe alone determines the geometry and final fate of the cosmos. In that case, there are three possibilities. If Ω is larger than one, then the universe would have a closed geometry and ultimately collapse in a Big Crunch. If it is equal to one, then the universe would have a flat geometry and would expand forever. If it is less than one, then the universe would have an open geometry and would also expand forever.