Nobody knows exactly what dark energy is, but most theoretical physicists agree that it comprises most of the energy in the universe and is largely responsible for the fact that the universe hasn't collapsed back in on itself like a deflating balloon. Or as SciShow host Hank Green puts it: "The bottom line is that whatever dark energy, and whatever it's doing, it's doing it like crazy." Actually, Hank's three-minute description of dark energy is well worth watching in its own right:
SpaceDaily has recently brought attention to the Basilakos-Solá hypothesis, which argues that the quantum vacuum itself, the apparent emptiness of the inert universe, produces the majority of the universe's energy (and the entirety of dark energy) by way of quantum fluctuations. The idea that there is more energy in the quantum vacuum than we suspect isn't an especially new one—Einstein's cosmological constant expressed a very similar idea—but the idea that dark energy could exist in the quantum vacuum not as a constant, but as a very large-scale distribution of quantum fluctuations, is a bold one. ("Nothing," Paula Solá told the magazine, "is more 'full' than the quantum vacuum, since it is full of fluctuations that contribute fundamentally to the values that we observe and measure.") Basilakos and Solá suggest that dark energy is simply the unrecorded energy crackling in and out of existence all around us—not a strange new form of energy, but an unrecognized expression of what we can already see.