Dark energy is the placeholder term for the unknown mechanism that has been causing accelerated expansion of the universe for about five billion years.

The theory suggests that dark energy permeates space and may drive the universe to expand or contract throughout its history. The model's flexibility allows it to fit every possible observed behavior of the universe's expansion, rendering it unfalsifiable and untestable.

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.

What is dark energy? What is dark matter? Well, if we knew exactly, we would have a Nobel prize—we know that they exist, though. Scientists have theorized that the former may be a property of three-dimensional space itself, while the latter could be exotic particles that interact with almost nothing. This video from Kurzgesagt explains what we know and what we don't.

As the universe expanded after the Big Bang, matter and energy diluted at different rates, with light once the most dense substance in the cosmos, followed by a period when matter was the most dense component. About five billion years ago, the density of dark energy was higher than that of both light and matter, which will continue to dilute.

The discovery, which came from analyzing extensive data from a major cosmological survey, challenges the assumed constancy of dark energy in modern physics. Understanding dark energy's behavior is crucial for predicting the universe's long-term evolution.

The Lambda cold dark matter model uses the Greek letter to denote the presence of a cosmological constant version of dark energy, alongside slow-moving dark matter and ordinary visible matter. It can account for the observed distribution of galaxies, the structure of the cosmic microwave background, the acceleration of the universe, and the abundance of hydrogen.

The Hubble tension refers to mismatches in the calculated rate of expansion of the universe derived from different methods, such as supernova data and the cosmic microwave background. By allowing dark energy to become active at a different time, models of the universe on which some of these methods depend are revised, altering the calculated values.

While the sun is expected to continue shining for another five billion years, the expansion of the universe will cause nearby stars and galaxies to become ever more distant, until they are too far to observe. Once the last old, red, dim stars cool into darkness after trillions of years, the universe will settle into a state of eternal, dark emptiness.

Physicists in the 1960s used their most successful theory to determine whether the accelerated expansion of the universe arises from quantum fluctuations in empty space. However, the mismatch between vacuum energy density and dark energy was so severe that it has been referred to as "the worst theoretical prediction in the history of physics."

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.

Though it makes up about 25% of the universe, dark matter can't be seen directly; its existence is inferred through its gravitational effects, like the unexpectedly fast rotation of galaxies. The concept began in 1933 but gained wide acceptance after Vera Rubin's galaxy rotation studies in the 1970s.

If the density of dark energy, which produces a repulsive effect through the expansion of space, increases with time, then all structures of matter, including galaxies, stars, planets, and atoms, will be pulled apart once the strength of dark energy surpasses the magnitude of the forces holding those structures together, including the gravitational and electromagnetic forces.

Primarily composed of visible matter, this celestial object would also contain a significant amount of dark matter that would annihilate with itself, converting its mass into energy. This energy would prevent the star from collapsing under its own gravity and from triggering nuclear fusion until its dark matter fuel was exhausted.

Research shows that while universes with slightly weaker dark energy form stars more efficiently, universes with stronger dark energy are statistically more common and can still support star formation. This challenges the anthropic logic that our universe must have ideal conditions for life because we exist to observe them.

About 95% of the observable universe is made of substances whose composition is unknown, but most of what remains (4%) is in the form of clouds of hydrogen and helium, followed by gases and plasma within stars (0.5%) and neutrinos created in nuclear reactions (0.3%).

One of the most surprising discoveries about our universe over the past century is that it's expanding. Even more surprising is that it’s speeding up, and scientists aren’t entirely sure why. The rate should be roughly dependent on the total energy contained in the universe, but calculations keep coming up short. Explore the so-called “Hubble Tension,” along with potential candidates to solve the riddle.

The amount of visible matter in the universe is most accurately determined by measuring the cosmic microwave background. Quantities of dark matter and dark energy are calculated through the observed bending of light around galaxies and the expansion of the universe, respectively.

As gravity tried to bring matter closer together in the early universe, its high temperature produced an outward pressure that caused it to expand. These competing forces caused matter to vibrate and produce the equivalent of sound waves, which spread outward as the universe expanded and cooled, serving as relics of dark energy's history.

Theories for what constitutes dark matter have ranged from slow, planet-sized objects to light subatomic particles that travel at high speeds. However, fast-moving candidates would quickly spread across the universe, preventing the gradual gathering of visible matter into the galaxies and galaxy clusters seen today through gravity.

The Dark Energy Spectroscopic Instrument is conducting a five-year redshift survey of over one-third of the sky, measuring the receding motions and locations of about 35 million galaxies and millions of stars using light captured by 5,000 optical fibers.

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.

Despite making up the vast majority of matter in the universe, dark matter can only be directed indirectly via its influence on other objects through gravity, the weakest of the fundamental forces. This means direct detection of dark matter may only be possible with future, more sensitive instruments.

The theories behind the Big Bang suggest the process of the universe beginning to the creation of stable, neutral atoms took just half a million years. But the transition from such atoms to star formation took surprisingly much longer, upwards of 100 million years. Why? This deep dive explores the science behind the unique physical composition of the so-called cosmic dark ages.

The distribution of matter in the universe is not uniform, with interconnected threads of galaxy clusters surrounded by voids of empty space. Simulations of the universe show this is only possible within 13.8 billion years—the universe's age—if a foundational network of seeds—dark matter halos—were present to gravitationally attract visible matter.

In high-energy collisions, exotic forms of matter can be produced by directly converting energy into mass according to E = mc². Even though a dark matter particle would not be directly observable, its creation would result in unaccounted-for mass, which can be compared to potential dark matter candidates for indirect detection.

A student-faculty team at Colgate University modeled the event as occurring months after the conventional Big Bang and determined the potential gravitational wave signal that would have been left behind. Future gravitational wave detectors are expected to be sensitive enough to observe this signal, if it exists.

Researchers from Yonsei University detected dark matter on the cosmic web, a network of filaments that feed gas into galaxies. The detection was made possible by using the general theory of relativity to analyze gravitational microlensing around the Coma Cluster. This article explains the phenomenon and how the detection supports the presence of dark matter throughout the cosmic web.

Among the possibilities for what makes up dark matter, some physicists champion axions, which could clump into galaxy-scale quantum states. These states would act as massive, unified superparticles that could explain the observational evidence for the universe's missing matter.

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.

As light rays travel near massive objects in space, their paths can be distorted on their way to Earth, causing objects to appear displaced, magnified, shrunken, or sheared. By simulating these effects for various mass distributions, astronomers can match the distortion to a simulated mass and compare it with the observed visible matter to infer the presence of dark matter.

Using data from the cosmic microwave background, which allows astronomers to model the universe's conditions since its earliest moments, yields a rate of expansion—the Hubble constant—lower than values obtained from multiple techniques measured locally at more recent timescales. Reconciling the discrepancy, known as the Hubble tension, will require a third independent measurement method between the two scales

Traditional physics breaks down at the moment of the Big Bang, but quantum theory suggests that “empty” space is never truly empty. In 1973, physicist Edward Tryon proposed that our universe began from a quantum energy blip in the quantum vacuum.

Just after the Big Bang, the high-density universe prevented light from traveling very far before being absorbed and emitted repeatedly by free-floating particles. As the universe expanded, electrons and protons combined to form hydrogen, thereby freeing up space for light to travel unimpeded, creating the background light we observe today.

While we have strong evidence that the universe expanded from an extremely hot, dense state, we can only trace events back to a tiny fraction of a second after that point. Earlier stages, like inflation and potential force unification, remain unproven and unobservable with current tools.

The former resulted from analyzing the motion of galaxies in the Coma Cluster, a subset of the approximately 30,000 galaxies he cataloged during his life. When hypothesizing an origin for observed high-energy particles called cosmic rays, Zwicky and colleague Walter Baade thought they could come from exploding stars, or supernovae—a term he coined—which would leave behind a collapsed star made entirely of neutrons.

The cosmic microwave background radiation and the observed expansion of the universe suggest that the cosmos began from a tiny, dense state, whose resulting properties we see throughout space. While physics allows for cyclical expansions and crunches, ongoing inflationary aeons, and localized inflation to create bubble universes, evidence for these alternatives has not yet been found.