The dark universe, encompassing dark matter and dark energy, is completely invisible to us, making it one the greatest mysteries of the cosmos. While our world appears mostly made up of atoms containing protons, neutrons, and electrons, surveys of the universe's contents have shown normal, visible matter (5%) is overwhelmingly eclipsed by dark matter (27%) and dark energy (68%).
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Dark matter holds galaxies and other large-scale structures together through gravity, and dark energy drives the universe's accelerating expansion. While scientists do not know what either is made of, they see how these dark components affect the normal matter in the universe.
Astronomers discovered that galaxies rotate in ways that visible matter alone cannot explain, pointing to the existence of unseen matter. According to current models, the universe has not been around long enough for gravity to have formed galaxies and other large-scale structures unless dark matter were present in quantities five to six times greater than visible matter.
Astronomers discovered in the late 1990s that galaxies move apart faster over time, contradicting expectations that gravity should slow cosmic expansion. This prompted the revival of the "cosmological constant," a form of repulsive gravity first proposed by Albert Einstein.
She discovered that stars on the outer edges of galaxies were orbiting much faster than expected based on visible matter alone, implying the presence of unseen mass exerting gravitational force. Without dark matter, the outer stars in galaxies would fly off into space instead of staying in orbit.
The theory of general relativity posits that mass and energy bend space and slow down the passage of time. One consequence is that massive objects, such as black holes, curve spacetime to such an extent that light from background sources can loop around them, producing multiple images or ring-like patterns.
In the 1920s, Hubble showed that the Andromeda "nebula" was another galaxy. He later discovered that galaxies move away from us at speeds proportional to their distance—a breakthrough now known as the Hubble–Lemaître law, foundational to Big Bang cosmology.
These explosions are all caused by exploding white dwarfs—remnants of dead, low-mass stars—and have the same peak brightness. By comparing them, astronomers have mapped the locations of galaxies alongside velocity data to determine the universe's expansion rate across time and the existence of dark energy.
General relativity predicted that the gravity of all the matter in the universe would cause it to collapse, contradicting the belief at the time that the universe was constant and unchanging. Albert Einstein added the cosmological constant to his equations, which acted like anti-gravity, to "fix" his math.
Theoretical candidates, such as sterile neutrinos, axions, and supersymmetric WIMPs, may open the door to a new branch of particle physics that shapes the cosmos through gravity. Some of these particles may constantly pass through us but interact too weakly to be detected.
Scientists using the Dark Energy Spectroscopic Instrument (DESI) analyzed light from 15 million galaxies and quasars to identify that the universe's accelerated expansion is diminishing. If this continues, the universe might reverse course and collapse in as little as 20 billion years.
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