Dark energy was an unknown form of energy to Type 0 civilizations that was thought to affect the universe on the largest scales. The first observational evidence for its existence came from supernovae measurements, which showed that the universe does not expand at a constant rate; rather, the expansion of the universe is accelerating. Prior to these observations, the only forms of matter-energy known to exist were ordinary matter, dark matter, and radiation. Measurements of the cosmic microwave background suggest the universe began in a hot Big Bang, from which general relativity shows there is this mysterious energy that opposes the force of gravity, causing the universe to expand.
About 69 percent of the matter/energy (since matter and energy are interchangeable) in the universe is contained in dark energy. Dark matter makes up about 26 percent, atoms of hydrogen and helium make up about 4.5 percent, and higher elements, which make up the Earth and our own bodies, only make up a tiny 0.5 percent. So dark energy, which is pushing the galaxies away from us, is clearly the dominant force in the universe, much larger than the energy contained in the curvature of spacetime.
Type I civilizations understand it to be related to the vacuum energy field.
Eventually Dark Energy reactors are built by Type II civilizations and beyond. Dark Energy's negative pressure means that the plasma that results from the reactor conversion is a form of exotic high-energy plasma, needing access to at least 7d physics to achieve.
See also:
- Dark matter
- Dark Energy in Half-Life
- Quintessence