Dark energy is a very sparse, uniform negative pressure that permeates the entire observable universe. It accounts for 70% of the mass/energy in the universe and is responsible for its accelerating rate of expansion. Dark energy is unlike the energy we are familiar with because it is not concentrated locally, as is the case with stars and galaxies, manifestations of conventional matter and energy. There are many other important differences between conventional energy and dark energy, which physicists continue to investigate.
The exact form or mechanism of operation of dark energy is unknown. In this respect, it is similar to its cousin, dark matter, which can only be observed by the influence it has on normal matter and energy.
There are two major theories for the form of dark energy, although one is more prominent than the other. The first theory, quintessence, describes the dark energy as a fluctuating field that changes its intensity based on location. The second theory, that of a cosmological constant, describes dark energy as constant and uniform. It is this second theory that is believed by most physicists and forms the basis of the Lambda-CDM model, the prevailing model of the structure of the cosmos.
The negative pressure of the cosmological constant is thought to originate from vacuum fluctuations at extremely small scales in all space. So-called virtual particles are continuously created and destroyed in this vacuum, creating a quantum foam that itself has energy.
The existence of dark energy has implications for the ultimate fate of the universe. If dark energy is an intrinsic property of space, as it looks to be, then it will continue to be exist indefinitely. If dark energy is the cause of the universe’s accelerating expansion, then it will also be the cause of reducing the average density of any parcel of space in the long run. As the universe grows more and more sparse, it will also grow more cold and hostile to life. Therefore, dark energy can justifiably be blamed for bringing on the “Heat Death” of the universe.