the cosmological constant
Einstein introduced it in 1917 as a fudge factor: he wanted a static universe, so he added a term to his equations that would balance gravity's pull and hold the cosmos still. When Hubble showed the universe was expanding, Einstein reportedly called it his biggest blunder and dropped it. But the term refused to die, and today it is our leading description of dark energy and the accelerating universe. Its symbol is the Greek letter Lambda.
The cosmological constant is a constant term Lambda that appears in Einstein's field equations, G_mu_nu + Lambda g_mu_nu = (8 pi G / c^4) T_mu_nu, where G_mu_nu is the Einstein tensor and g_mu_nu the metric. Moved to the other side, it acts exactly like an energy density of empty space, rho_Lambda = Lambda c^2 / (8 pi G), with pressure p = minus rho_Lambda c^2, so its equation-of-state parameter is precisely w = minus 1. Unlike matter, this energy does not dilute as space expands; every cubic metre of vacuum carries the same energy, so as the universe grows, Lambda comes to dominate and drives exponential expansion.
The cosmological constant is the simplest candidate for dark energy and fits the data beautifully, forming the Lambda in the standard Lambda-CDM model. But it hides the deepest puzzle in physics: quantum field theory says empty space should teem with zero-point energy, and a naive calculation gives a vacuum energy roughly 10^120 times larger than the observed Lambda. Why the real value is so absurdly small, yet not exactly zero, is the cosmological constant problem, unsolved and pointing squarely at the missing theory of quantum gravity.
In the standard Lambda-CDM model, the cosmological constant contributes about 68 percent of the energy budget, cold dark matter about 27 percent, and ordinary matter about 5 percent. That single number Lambda, fitted to the data, reproduces the CMB, the expansion history, and the growth of structure all at once.
Lambda: one number that carries most of the universe's energy budget.
The cosmological constant is a specific model of dark energy (the case w exactly minus 1); dark energy is the broader observed phenomenon. That we can measure Lambda precisely yet cannot compute its value from first principles is one of physics' great unsolved problems.