Dark energy is the name given to an unexplained aspect of the universe. Dark energy has been postulated as an explanation for the accelerating motion of galaxies away from each other.
Scientists theorize that galaxies have been moving away from each other since the time of the Big Bang. This is the expansion of the universe originally triggered by the Big Bang that just keeps going due to inertia.
One would think that, over the eons, the gravity of galaxies pulling on each other would have slowed this expansion. Instead, scientists have found that the expansion is speeding up. They postulate that an unknown form of energy, “dark energy,” pushes outward and causes this acceleration. Dark energy is thought of as countering gravity and, further, overbalancing it.
In accordance with E=mc^2, energy is equivalent to mass, so dark energy would add to the mass of the universe. Estimates vary, but dark energy could account for 68% of the energy in our universe.
Dark energy and dark matter are different. While dark energy has been postulated as an explanation for the accelerating motion of galaxies away from each other, “dark matter” is the name given to another unexplained aspect of the universe. Dark matter could account for “excess gravity” in the universe, that is, stronger gravity than can be accounted for by scientists’ calculations of the amount of matter that they can see in the universe.
Dark energy in relation to vacuum energy. The concept of dark energy may relate to the concept of vacuum energy. The two terms may name the same thing. But when vacuum energy is calculated, it is 120 orders of magnitude more forceful than is needed to explain the accelerating expansion of the universe. So, the magnitude of the two types of postulated energy don’t fit together well, to say the least.