Dark energy is the name given in cosmology to the unknown component invoked to explain the accelerating expansion of the universe. Its existence is inferred from astronomical observations rather than from direct laboratory detection. In the standard Lambda–cold dark matter model (ΛCDM), it takes the form of a cosmological constant: a uniform, persistent energy density associated with space itself. Other explanations involve evolving fields or changes to the theory of gravity. The observational evidence for acceleration does not, by itself, establish which explanation is correct. (science.nasa.gov)
Cosmic abundance and distinction from dark matter
Within ΛCDM, dark energy accounts for approximately 68–70 percent of the present cosmic mass–energy budget. Dark matter contributes roughly 25–27 percent, and ordinary matter about 5 percent. These proportions are inferred through cosmological models fitted to observations; they are neither percentages of the universe’s volume nor fixed proportions throughout cosmic history. (esa.int)
Dark matter and dark energy have different roles. Dark matter clusters gravitationally and helps form the structures in which galaxies develop. Dark energy, in its simplest formulation, remains spatially uniform and affects the universe’s large-scale expansion. “Dark” therefore denotes an observational unknown, not a shared substance or mechanism. (science.nasa.gov)
Discovery of accelerated expansion
The decisive evidence emerged in 1998 from two independent teams studying distant Type Ia supernovae. Their peak luminosities can be standardized using measured properties of their light curves, allowing astronomers to estimate distances. Comparing these distances with redshifts reconstructs the expansion history: distant explosions were fainter, and therefore farther away, than expected in the matter-dominated, decelerating models under consideration. (nobelprize.org)
The Supernova Cosmology Project and the High-Z Supernova Search Team concluded that cosmic expansion was accelerating. Saul Perlmutter, Brian Schmidt, and Adam Riess received the 2011 Nobel Prize in Physics for this discovery. Subsequent measurements using independent methods strengthened the evidence, reducing the possibility that an unrecognized supernova effect alone explained the observations. (nobelprize.org)
Pressure and cosmic dynamics
In general relativity, part of relativity theory, both energy density and pressure influence the evolution of spacetime. For a homogeneous, isotropic universe, the acceleration equation can be written
where is the cosmic scale factor, the gravitational constant, the speed of light, the total energy density, and the total pressure. A cosmological constant is included here as an effective fluid. Acceleration occurs when . Thus sufficiently negative pressure can produce accelerated expansion without requiring negative energy density. (arxiv.org)
A component’s equation of state is often expressed as . A positive cosmological constant has . Its density stays constant as space expands, whereas nonrelativistic matter density decreases as . Consequently, matter dominated earlier epochs, while dark energy became relatively more important later. Acceleration means ; it does not necessarily mean that the Hubble parameter increases with time. (arxiv.org)
Principal explanations
The simplest explanation is the cosmological constant, represented by Λ in the Einstein field equations. Albert Einstein introduced it in 1917 while constructing a static cosmological model. In modern cosmology, a positive Λ instead provides an economical description of accelerated expansion. It introduces no independently evolving dark-energy field. (arxiv.org)
Another possibility is quintessence, a hypothetical evolving scalar field. Its balance of kinetic and potential energy determines its pressure and density, allowing to vary with time. No particular quintessence field has been observationally identified. Alternatively, modified-gravity theories attempt to explain acceleration by changing gravity’s behavior on cosmic scales rather than adding a new energy component. These possibilities require tests of both expansion and the growth of structure. (science.nasa.gov)
Observational tests and evolving dark energy
Different observations constrain complementary aspects of the problem. Baryon acoustic oscillations provide a calibrated distance scale in the distribution of galaxies. The cosmic microwave background supplies information about early-universe conditions and geometry. Weak gravitational lensing measures matter distributions and their evolution. Combining these with supernova distances helps distinguish dark-energy behavior from other cosmological parameters. (pdg.lbl.gov)
On March 19, 2025, the Dark Energy Spectroscopic Instrument collaboration reported results from its first three years of observations. DESI measurements alone remained consistent with ΛCDM, but combinations with other datasets favored a time-varying dark-energy model over a cosmological constant. The reported preference ranged from 2.8 to 4.2 standard deviations, depending on the datasets combined. These were indications rather than a definitive discovery: their interpretation depends on statistical assumptions, the chosen parameterization, and possible systematic errors. (newscenter.lbl.gov)
Vacuum energy and unresolved questions
A cosmological constant can be interpreted as vacuum energy. In quantum field theory, fields can contribute energy even in their lowest-energy states. However, known contributions and straightforward theoretical estimates do not naturally reproduce the very small value inferred cosmologically. Explaining this discrepancy is the cosmological constant problem, not an established calculation of dark energy’s origin. (arxiv.org)
A separate “coincidence problem” asks why matter and dark-energy densities are comparable during the present epoch despite evolving differently. Long-term predictions also depend on the underlying explanation: a persistent positive Λ leads toward increasingly vacuum-dominated expansion, whereas evolving dark energy can produce different futures. Measurements over the observed expansion history do not uniquely determine its behavior arbitrarily far into the future. (arxiv.org)