General relativity predicts that if the universe is below the critical density, it will expand forever and have a hyperbolic geometry. In contrast, a universe in which the density of matter is greater than W critical has a negative total energy. Meanwhile, a critical-density universe has zero energy. Einstein's theory identifies the energy of the universe with the nature of its geometry. Only a critical-density universe is Euclidean.
Observations strongly hint that the matter density is subcritical.
A variety of techniques point to a matter density that is one third of the critical value, with an uncertainty of a factor of two at the most. However, we cannot infer that the universe is destined to expand forever. The concept of dark energy was originally introduced by Einstein in 1917, some 12 years before Edwin Hubble discovered that the universe is expanding. Einstein conceived the idea to counter the gravitational effect of matter and provide a static universe that neither collapsed nor expanded. He called this repulsive force the cosmological constant, which appeared as an additional constant in the equations of general relativity and had no Newtonian counterpart.