What Is Dark Matter and Dark Energy?

Key Takeaways

• Dark matter and dark energy together account for roughly 95% of the universe’s total energy budget
• Vera Rubin’s rotation curve data in the 1970s gave dark matter its strongest observational foundation
• No confirmed direct detection of a dark matter particle or dark energy source has occurred as of 2025

Dark Energy and Dark Matter by the Numbers

In 1998, two independent research teams studying distant Type Ia supernovae reported a finding that inverted decades of cosmological expectation: the universe was not just expanding, it was expanding faster and faster. That discovery eventually earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics, and it thrust dark energy into the center of modern cosmology. The combined study of dark energy and dark matter now defines one of the most active and unresolved problems in all of science.

According to measurements from the Planck satellite, which completed its mission in 2013 after mapping the cosmic microwave background (CMB) with unprecedented precision, the universe’s composition breaks down as follows: approximately 68% dark energy, 27% dark matter, and roughly 5% ordinary matter. That means everything visible to telescopes, every star, planet, nebula, and galaxy, adds up to just 5% of what exists. The remaining 95% neither emits nor reflects light in any detectable way.

The standard model for describing the universe’s large-scale structure is called Lambda-CDM, where Lambda represents the cosmological constant associated with dark energy and CDM stands for cold dark matter. This model has survived decades of observational testing and remains the most accurate framework available for predicting how galaxies cluster, how the universe expands, and how structure formed after the Big Bang. It does not explain what dark energy and dark matter actually are at a fundamental level.

NASA’s dedicated overview of dark energy and dark matter describes dark energy as the name assigned to the unknown cause of the universe’s accelerating expansion, while dark matter refers to an unseen form of mass whose gravitational effects are observed across many different scales. Neither term describes a confirmed substance. Both describe confirmed behaviors that current physics cannot fully account for.

Fritz Zwicky, the Coma Cluster, and the Original Inference

The phrase “dark matter” traces to Fritz Zwicky, a Swiss astronomer working at the California Institute of Technology who, in 1933, applied the virial theorem to the Coma Cluster, a dense collection of hundreds of galaxies roughly 320 million light-years from Earth. Zwicky calculated the cluster’s total mass by measuring how fast its member galaxies were moving relative to one another. The velocities he observed implied a mass far greater than the luminous matter visible through the telescope could account for. He proposed that an unseen form of mass, which he called “dunkle Materie” in German, was providing the additional gravitational pull.

Zwicky’s work went largely unrecognized for decades. The tools available in the 1930s were limited, and the idea that most of the universe’s mass might be invisible was too speculative to gain traction quickly. It wasn’t until the 1970s and 1980s, when a new generation of astronomers brought better instrumentation and more systematic observational programs to bear, that the evidence became too consistent to dismiss.

The Coma Cluster itself remains one of the most studied structures in the sky. Modern estimates of its mass, derived from gravitational lensing and X-ray observations of hot gas between the galaxies, continue to confirm a large discrepancy between visible and total mass. Zwicky’s original insight, reached through a comparatively crude calculation, turned out to be directionally correct even if the specifics have been refined substantially over the intervening nine decades.

The idea that a galaxy cluster could be held together by something invisible also raised deeper questions about whether the physical laws applied on Earth behave uniformly at very large scales. If gravity behaves differently over enormous distances, that could explain the discrepancy without requiring unseen mass. That alternative, known as Modified Newtonian Dynamics, was proposed by Mordehai Milgrom in 1983 and has attracted a persistent minority of supporters, though most physicists consider the dark matter interpretation better supported by the totality of evidence available.

How Vera Rubin’s Galaxy Rotation Measurements Changed Physics

Vera Rubin, an astronomer at the Carnegie Institution of Washington, spent much of the 1970s measuring the rotational velocities of stars and gas within individual spiral galaxies. The expectation, based on Newton’s laws, was straightforward: just as planets closer to the Sun orbit faster and outer planets orbit more slowly, stars in the outer regions of a galaxy should move slower than stars near the center. The visible mass is concentrated toward the middle, so the gravitational pull should weaken with distance.

That is not what Rubin found. Working with Kent Ford, who had developed a highly sensitive spectrograph, she measured galaxy rotation curves for dozens of spiral galaxies, including the Andromeda Galaxy. The curves were flat. Stars in the outer arms moved at roughly the same speed as stars much closer to the galactic center, even at distances where the visible mass should no longer exert much pull.

The only consistent explanation was that each galaxy was embedded in a much larger halo of unseen mass, one that extended well beyond the visible disk and provided enough gravitational influence to keep the outer stars moving at the speeds observed. Rubin published her findings through the late 1970s and into the 1980s, accumulating data on more than 200 galaxies. The pattern held across every galaxy she and her collaborators studied.

Rubin’s contribution differed from Zwicky’s in an important way. While Zwicky had looked at a cluster of galaxies and noticed a mass discrepancy at a very large scale, Rubin found the same problem playing out within individual galaxies, structures that astronomers had long studied in detail. The implication was unavoidable: dark matter was not an exotic property of extreme environments. It was a feature of virtually every galaxy ever examined.

Her work, combined with Zwicky’s earlier inference and theoretical modeling of large-scale structure formation, pushed dark matter from the realm of speculation into the mainstream of astrophysics. Rubin was nominated for the Nobel Prize repeatedly but never received it before her death in 2016.

Direct and Indirect Evidence That Dark Matter Is Real

Beyond galaxy rotation curves, several independent lines of evidence point to the existence of dark matter. The Bullet Cluster, a system formed by the collision of two galaxy clusters roughly 3.7 billion light-years away, has provided what many physicists regard as the most direct observational evidence to date. When two clusters collide, the hot gas in each cluster, which contains most of the ordinary matter, slows down due to electromagnetic interaction. But dark matter, which interacts only through gravity, passes through essentially unimpeded. Observations combining X-ray data from the Chandra X-ray Observatory and gravitational lensing maps showed that the majority of the mass in the merged cluster was spatially separated from the ordinary matter, exactly as dark matter theory predicts.

The cosmic microwave background (CMB) provides another form of evidence. Dating back to roughly 380,000 years after the Big Bang, this radiation is the faint thermal relic of the universe’s first transparent moment, and its temperature fluctuations carry detailed information about the early universe’s composition. Measurements made with high precision by the Wilkinson Microwave Anisotropy Probe between 2001 and 2010, and then by the Planck satellite from 2009 to 2013, are consistent with a universe containing a substantial amount of non-baryonic cold dark matter; models built without dark matter cannot reproduce the observed pattern.

Gravitational lensing, where massive objects bend light from more distant sources, has been used to map the distribution of dark matter across the sky without any assumption about what it is physically made of. These maps consistently show dark matter concentrated in halos around galaxies and in filamentary structures connecting galaxy clusters, a pattern that matches the predictions of Lambda-CDM.

None of this evidence tells scientists what dark matter is. It defines what dark matter does. The distinction matters enormously for experimental physics, because a successful detector must be calibrated to look for something specific, and “something with gravity but no electromagnetic interaction” is not a precise enough description to build an experiment around.

The Leading Theoretical Candidates for Dark Matter and Dark Energy

The most extensively studied dark matter candidate has been the weakly interacting massive particle (WIMP). WIMPs are hypothetical particles that interact through gravity and the weak nuclear force but not through electromagnetism, making them invisible to conventional telescopes. Their predicted mass range is broadly consistent with what particle physics theories such as supersymmetry suggest for undiscovered particles. The appeal of WIMPs historically came partly from what physicists called the “WIMP miracle”: the observation that a particle with roughly the right properties to be produced in the early universe would naturally end up with roughly the right abundance to account for the observed dark matter density today.

Underground detectors have searched for WIMPs for decades. The XENON1T experiment, housed deep beneath Italy’s Gran Sasso mountain, used 3.5 tonnes of liquid xenon to search for the faint recoil signal that would result if a WIMP collided with a xenon nucleus. After years of operation, XENON1T found no confirmed signal. Its successor, XENONnT, continues the search with a larger detector. The Large Hadron Collider at CERN has also searched for WIMP-like particles through high-energy proton collisions without producing a confirmed detection.

Axions represent a different class of dark matter candidate. Originally proposed in the late 1970s by Roberto Peccei and Helen Quinn to solve a separate problem in quantum chromodynamics known as the strong CP problem, axions would be extraordinarily light, far lighter than WIMPs, and would interact with photons under the influence of a strong magnetic field. The Axion Dark Matter Experiment at the University of Washington has been searching for axion signals since the late 1990s and has progressively narrowed the range of axion masses it can rule out, without yet finding a confirmed signal.

For dark energy, the leading theoretical interpretation remains the cosmological constant, originally introduced by Albert Einstein in 1917 as a term in his field equations of general relativity. Einstein later called it “his greatest blunder” after Edwin Hubble’s observations confirmed the universe was expanding. After 1998, the cosmological constant was revived as the most straightforward mathematical description of an energy density that is uniform across space and constant through time, consistent with how the observed acceleration behaves. The problem is that quantum field theory predicts a vacuum energy density roughly 120 orders of magnitude larger than what cosmological observations imply, a discrepancy with no accepted resolution.

An alternative framework is quintessence, a hypothetical dynamic energy field that varies in space and time. Unlike the cosmological constant, quintessence would have a slightly different effect on the expansion rate at different epochs of the universe’s history, which future surveys could potentially detect and distinguish from a true constant.

Why Direct Detection Has Remained Out of Reach

The difficulty in detecting dark matter particles comes down to the nature of the interaction being sought. If dark matter particles pass through ordinary matter with only the weakest possible interactions, a detector the size of a building might register only a handful of events per year, and those events would look nearly identical to background noise from radioactive decay or cosmic ray interactions. Experiments like LUX-ZEPLIN, located at the Sanford Underground Research Facility in South Dakota, and PandaX, housed in the China Jinping Underground Laboratory, must be built deep underground to shield against cosmic rays and must use extremely pure materials to minimize internal radioactivity.

Decades of null results have not disproven dark matter’s existence. They have ruled out a large portion of the parameter space that was considered most theoretically attractive, particularly for heavier WIMPs. This has prompted some physicists to explore lighter candidates, including sterile neutrinos and primordial black holes, the latter being dense objects formed in the early universe rather than from stellar collapse.

Dark energy presents a different kind of detection problem. It appears to be a property of space itself, distributed uniformly throughout the universe, meaning it can’t be collected, concentrated, or isolated in a laboratory. Measurements of its effects come exclusively from astronomical observations: the rate at which the universe expands, the distribution of galaxies at different distances, and the way the growth of large-scale structure has slowed or accelerated at different epochs. The Dark Energy Survey, which concluded its six-year observing program in January 2019, mapped hundreds of millions of galaxies to constrain dark energy’s properties. The Euclid space telescope, launched by the European Space Agency in July 2023, is designed to map the geometry of the universe over roughly one-third of the sky, providing the most detailed picture yet of how dark energy has influenced cosmic structure over billions of years.

Summary

Dark matter and dark energy together account for 95% of the universe’s energy and mass content, yet neither has been directly detected or definitively identified. The observational case for dark matter, built from galaxy rotation curves, cluster dynamics, the Bullet Cluster, CMB measurements, and gravitational lensing, is strong and consistent across many independent methods. The case for dark energy rests primarily on the measured acceleration of cosmic expansion, confirmed by multiple teams and methods since 1998.

What neither field has achieved, despite decades of investment and increasingly sensitive experiments, is a confirmed particle or direct measurement. XENON, LUX-ZEPLIN, the Axion Dark Matter Experiment, and the Large Hadron Collider have narrowed the search without ending it. The Vera C. Rubin Observatory, named in honor of the astronomer whose rotation curve work shaped the field, and missions like Euclid may eventually provide enough precision to distinguish between a true cosmological constant and a dynamic energy field. For now, the universe’s dominant constituents remain known in their effects and unknown in their nature.

Appendix: Top 10 Questions Answered in This Article

What percentage of the universe is made up of dark matter and dark energy?

According to Planck satellite data, dark energy accounts for approximately 68% of the universe’s total energy content, while dark matter makes up roughly 27%. Ordinary matter, the material that forms all visible stars, planets, and galaxies, represents only about 5% of the total.

Who first proposed the concept of dark matter?

Fritz Zwicky, a Swiss astronomer at the California Institute of Technology, introduced the concept in 1933 after applying the virial theorem to the Coma Cluster. His calculations showed the cluster’s visible mass was far too small to explain the observed velocities of its member galaxies, leading him to propose the existence of unseen mass.

What is the Lambda-CDM model?

Lambda-CDM is the standard cosmological model used to describe the universe’s large-scale structure and evolution. Lambda refers to the cosmological constant representing dark energy, and CDM stands for cold dark matter. The model successfully predicts galaxy clustering, the cosmic microwave background pattern, and the overall expansion history of the universe.

What did the Bullet Cluster reveal about dark matter?

The Bullet Cluster is a system formed from two colliding galaxy clusters. Observations showed that ordinary matter slowed during the collision while the gravitational mass continued moving forward, spatially separating the two components. This separation matched predictions for dark matter and is widely cited as among the most compelling observational evidence for its existence.

What are WIMPs and why do physicists favor them as dark matter candidates?

Weakly interacting massive particles are hypothetical particles that interact through gravity and the weak nuclear force but not through electromagnetism. Physicists historically favored WIMPs because the density they would produce in the early universe matches the observed dark matter abundance closely, a coincidence known informally as the WIMP miracle.

What is dark energy and what evidence supports its existence?

Dark energy is the label given to the unknown cause of the universe’s accelerating expansion, first confirmed in 1998 through observations of distant Type Ia supernovae by teams led by Saul Perlmutter, Brian Schmidt, and Adam Riess. Subsequent measurements from the cosmic microwave background and large-scale structure surveys have strengthened the case considerably.

What is the cosmological constant and how does it relate to dark energy?

The cosmological constant, originally introduced by Albert Einstein in 1917, is a term in the equations of general relativity representing a constant energy density of space. After 1998, it became the most widely accepted mathematical description of dark energy. Quantum field theory predicts a far larger value than observations imply, creating an unresolved theoretical puzzle.

What experiments are currently searching for dark matter particles?

Several experiments are actively searching, including XENONnT in Italy, LUX-ZEPLIN in South Dakota, and PandaX in China, all of which use large masses of shielded material to detect rare particle collisions. The Axion Dark Matter Experiment at the University of Washington searches for axions specifically, while the Large Hadron Collider at CERN searches for new particles that could match dark matter candidates.

Why has Vera Rubin’s work remained so significant in dark matter research?

Vera Rubin’s systematic measurements of galaxy rotation curves across more than 200 galaxies demonstrated that stars at the outer edges of galaxies move at speeds inconsistent with the distribution of visible mass. Her findings showed that dark matter was not confined to extreme environments but was a consistent feature of virtually every spiral galaxy studied.

What is the Euclid space telescope and what role does it play in dark energy research?

Launched by the European Space Agency in July 2023, Euclid is designed to map the shapes and positions of billions of galaxies across roughly one-third of the sky. By analyzing how dark energy has influenced the growth of cosmic structure over time, the mission is intended to distinguish between the cosmological constant interpretation of dark energy and alternative theories like quintessence.

Appendix: Glossary of Key Terms

Type Ia Supernova

A specific class of stellar explosion that occurs when a white dwarf in a binary system accumulates enough mass to trigger thermonuclear runaway. Because all Type Ia supernovae reach a similar intrinsic brightness, they serve as standard candles for measuring vast cosmic distances, making them essential tools for studying the expansion rate of the universe.

Cosmic Microwave Background

The faint thermal radiation permeating all of space, originating from roughly 380,000 years after the Big Bang when the universe cooled enough for hydrogen atoms to form and light to travel freely. Its temperature fluctuations encode detailed information about the composition, geometry, and early structure of the universe.

Lambda-CDM

The standard scientific model describing the universe’s composition and large-scale structure. Lambda represents the cosmological constant associated with dark energy, and CDM stands for cold dark matter, meaning dark matter particles that moved slowly relative to light in the early universe. The model is the most predictively accurate framework in modern cosmology.

WIMP

An acronym for weakly interacting massive particle, a hypothetical class of dark matter candidate. WIMPs would have masses in a range suggested by particle physics theories and would interact with ordinary matter only through gravity and the weak nuclear force, making them extraordinarily difficult to detect with conventional instruments.

Axion

A hypothetical very low-mass particle originally proposed to solve a problem in quantum chromodynamics called the strong CP problem. Because axions interact with photons in the presence of a strong magnetic field, specialized resonant cavity detectors have been designed to search for them as candidates for the dark matter that fills the universe.

Cosmological Constant

A term introduced into Einstein’s field equations of general relativity, represented by the Greek letter Lambda. Originally intended to allow for a static universe, it was abandoned and later revived after 1998 as the simplest mathematical description of dark energy. It represents a constant energy density uniformly distributed throughout space.

Galaxy Rotation Curve

A plot of the orbital velocities of stars and gas in a galaxy against their distance from the galactic center. Standard gravitational theory predicts that velocities should decrease with distance beyond the visible mass concentration. Flat rotation curves, observed by Vera Rubin and others, indicate the presence of additional unseen mass extending far beyond the visible disk.

Gravitational Lensing

A phenomenon predicted by general relativity in which massive objects bend the path of light passing nearby, distorting the apparent positions and shapes of background sources. Astronomers use gravitational lensing to map the distribution of mass, including dark matter, without requiring any assumption about what that mass physically is.

Quintessence

A hypothetical dynamic energy field proposed as an alternative explanation for dark energy. Unlike the cosmological constant, which remains fixed in time and space, quintessence would vary across both, producing a slightly different expansion history that future precision surveys could potentially detect and distinguish from a true constant.

Virial Theorem

A principle in classical and statistical mechanics relating the average kinetic energy of particles in a bound system to their average potential energy. Fritz Zwicky applied it to galaxies in the Coma Cluster to infer the cluster’s total mass from observed velocities of member galaxies, revealing a mass far exceeding what luminous matter could provide.