A poorly understood (and non-luminous) substance that exerts a pressure that tends to accelerate the expansion of the universe. This dark energy counteracts gravity's natural tendency to slow the expansion of the universe. It is particularly important to understand this substance, as it appears to make up the majority of our universe. Dark energy is also often referred to as the "cosmological constant". It is distinct from dark matter, which gravitates in the same way as ordinary matter, but is not luminous.