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Dark energy

Dark energy is a mysterious form of energy that dominates the universe and drives its accelerating expansion. Scientists discovered dark energy in 1998 when studying distant supernovae and found that the universe is expanding faster than expected, contrary to the prediction that gravity should slow it down. Today, dark energy makes up most of the universe's total mass and energy content. Its exact nature remains unknown, but the leading explanation is that it behaves as a cosmological constant—a uniform energy that fills all of space. Dark energy matters because understanding it is crucial to explaining the universe's past, present, and ultimate fate, and it represents one of the biggest mysteries in modern physics.

Sources

• en.wikipedia.org: “In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on its largest scales. Its primary effect is to drive the accelerating expansion of the universe.”
• en.wikipedia.org: “Assuming that the lambda-CDM model of cosmology is correct, dark energy dominates the universe, contributing 68% of the total mass-energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible.”
• en.wikipedia.org: “The first observational evidence for dark energy's existence came from measurements of supernovae. Type Ia supernovae have constant luminosity, which means they can be used to accurately measure distances. Comparing this distance to the redshift (which measures the speed at which the supernova is receding) shows that the universe's expansion is accelerating.”
• en.wikipedia.org: “The exact nature of dark energy remains a mystery, and many possible explanations have been theorized. The main candidates are a cosmological constant (representing a constant energy density filling space homogeneously) and scalar fields (dynamic quantities having energy densities that vary in time and space) such as quintessence or moduli.”