Dark energy makes up roughly 68% of the universe and is forcing it to expand at an accelerating rate.

The discovery of dark energy dates back to 1998 when two independent teams of astronomers led by Saul Perlmutter and Adam Riess observed distant Type Ia supernovae. These researchers found that the light from these exploding stars was fainter than expected, indicating they were further away than predicted by a decelerating universe model. This groundbreaking observation proved that the expansion of the universe is actually speeding up rather than slowing down due to gravity.Current data from the Planck satellite mission and the Wilkinson Microwave Anisotropy Probe (WMAP) estimate that dark energy accounts for approximately 68.3% of the total energy density of the universe. Dark matter makes up about 26.8%, while normal 'baryonic' matter—the stuff that makes up stars and planets—comprises only about 4.9%. This composition suggests that the vast majority of our universe is made of substances we cannot see or directly detect.Physicists often describe dark energy as the 'cosmological constant,' a term originally proposed by Albert Einstein in his General Theory of Relativity. While Einstein initially added it to keep the universe static, modern cosmologists use it to explain the vacuum energy of space. As space expands, the density of dark energy remains constant, which leads to an exponential increase in the total amount of repulsive force in the cosmos.This accelerating expansion has profound implications for the ultimate fate of the universe. If dark energy continues to dominate, the universe may eventually end in a 'Big Freeze' or 'Heat Death' where galaxies become so isolated that they can no longer be seen from one another. Scientists continue to study this phenomenon through projects like the Dark Energy Survey (DES) to determine if the strength of dark energy changes over time.