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 is responsible for keeping galaxies and other large-scale structures intact 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 dark matter. According to current models, forming galaxies like the Milky Way would be impossible without it.
Astronomers discovered in the late 1990s that galaxies move apart faster over time, contradicting expectations that gravity should slow cosmic expansion. This has prompted the revival of the "cosmological constant," a type of repulsive gravity theorized 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.
Scientists created the first 3D map of dark matter filaments, confirming theories that gravity pulled early density variations into a sprawling web that connects galaxies and clusters across billions of light-years. The algorithm used to make the map was modeled on slime mold's filament-building capabilities.
The theory of general relativity, widely regarded as Einstein's most significant accomplishment, posits that mass and energy bend space and slow down time. One consequence is that massive objects, such as black holes, bend spacetime so much that light coming from behind the object wraps back around it.
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 redshift data to determine the universe's expansion rate across time.
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, like sterile neutrinos, axions, and WIMPs from supersymmetry, 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 for detection.
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