What is dark matter in simple terms?

Dark matter is matter that we cannot see because it does not give off or block light, but whose gravity we can measure. It holds galaxies and galaxy clusters together and outweighs ordinary matter by about five to one.

Has dark matter ever been detected directly?

Not as of 2025. Its gravitational effects are measured in many independent ways, but underground detectors and particle colliders have not yet caught a dark-matter particle. Each experiment rules out some possibilities and narrows the search.

What is the difference between dark matter and dark energy?

Dark matter pulls things together through gravity and clumps around galaxies. Dark energy is spread smoothly through space and drives the accelerating expansion of the universe. They make up about 27% and 68% of the universe's energy content respectively.

Who discovered dark matter?

Fritz Zwicky first proposed unseen matter in the Coma Cluster in 1933. Vera Rubin and Kent Ford's measurements of galaxy rotation in the 1970s provided the evidence that convinced most astronomers it was real.

Could dark matter be black holes?

Ordinary black holes formed from dead stars cannot account for it, because there are not enough of them. Primordial black holes formed in the early universe remain a possibility for some mass ranges, but observations have ruled out many of them as the main ingredient.

The invisible scaffolding that holds galaxies together

Dark Matter

Invisible mass · Everywhere

Dark matter is the name astronomers give to a form of matter that does not emit, absorb or reflect light, yet makes its presence felt through gravity. Stars at the edges of galaxies orbit too fast, clusters of galaxies hold together when they should fly apart, and light from distant objects bends more than visible matter alone can explain. All of these clues point to something unseen.

According to measurements of the cosmic microwave background, dark matter makes up about 27% of the energy content of the universe and roughly 85% of all its matter. Ordinary matter, everything made of atoms, is only about 5%. Nobody yet knows what dark matter is made of, which makes it one of the biggest open questions in physics.

Key facts

Share of the universe: ≈ 27%Of the total energy content (Planck 2018)
Share of all matter: ≈ 85%About 5 times as much as ordinary matter
Ordinary matter: ≈ 5%Atoms: stars, gas, planets, people
First proposed for clusters: 1933Fritz Zwicky, Coma Cluster
Rotation-curve evidence: 1970sVera Rubin and Kent Ford
Interaction with light: None detectedRevealed only by its gravity
Sun's orbital speed: ≈ 230 km/sAround the Milky Way's centre
Mapping mission: Euclid (ESA)Launched 1 July 2023

Galaxies that spin too fast

In the Solar System, distant planets orbit more slowly than nearby ones because most of the mass sits in the Sun. Astronomers expected spiral galaxies to behave the same way: most of their visible stars and gas are concentrated towards the centre, so stars far out should move more slowly. A graph of orbital speed against distance, called a rotation curve, should rise and then fall.

Instead, measurements show rotation curves that rise and then stay roughly flat far beyond the visible disc. Stars and gas clouds at the edges move almost as fast as those closer in. If only the visible matter were present, those outer stars would be flung away. The simplest explanation is that each galaxy sits inside a much larger, roughly spherical halo of unseen matter that supplies the extra gravity.

Vera Rubin and Kent Ford

In the 1970s Vera Rubin and Kent Ford used a sensitive spectrograph to measure how fast stars and gas move across the Andromeda Galaxy and then dozens of other spirals. Their flat rotation curves, together with radio measurements of hydrogen gas by other astronomers, convinced most of the field that galaxies contain far more mass than we can see. The Vera C. Rubin Observatory in Chile is named in her honour.

Seeing it in the explainer

The 3D view lets you switch a spiral galaxy between visible matter only and visible matter plus a dark-matter halo. Without the halo, the outer stars lag behind and the rotation curve drops; with it, the curve flattens out, matching what telescopes actually observe.

Discovery: from Zwicky to the cosmic microwave background

In 1933 the Swiss astronomer Fritz Zwicky measured the speeds of galaxies in the Coma Cluster, about 320 million light-years away. They were moving so fast that the cluster's visible mass could not hold them together. He concluded that it must contain a large amount of unseen 'dark matter'. His figures were rough and the idea was largely set aside for decades.

Rotation curves revived the problem in the 1970s, and later evidence piled up from many directions. X-ray telescopes found clusters filled with hot gas that would escape without extra gravity. Maps of the cosmic microwave background showed that the pattern of ripples in the early universe needs matter that does not interact with light. Computer simulations found that galaxies and the cosmic web of filaments form in the right way only if dark matter is included.

Gravitational lensing and the Bullet Cluster

Einstein's general relativity predicts that mass bends the path of light. A massive galaxy cluster acts like a lens, stretching the images of more distant galaxies into arcs and distorting their shapes slightly. By measuring these distortions, astronomers can map where the mass in a cluster really is, whether or not it shines. Lensing maps consistently show far more mass than the stars and gas contain.

The Bullet Cluster is a famous example. It is the aftermath of two galaxy clusters that collided and passed through each other. X-ray images show the hot gas, which holds most of the ordinary matter, slowed down and piled up in the middle. Lensing reveals that most of the mass has moved ahead with the galaxies, separated from the gas. That is what you would expect if most of the mass were dark matter that barely interacts, and it is hard to explain by simply modifying the law of gravity.

What could dark matter be?

Dark matter is not simply dim stars, cold gas or rogue planets. Surveys of these objects find too few, and measurements of the early universe limit how much of the total can be made of ordinary atoms. Neutrinos exist and are 'dark', but they are too light and fast to build galaxies in the observed way. Most physicists therefore suspect a new kind of particle not included in the Standard Model of particle physics.

Leading candidates include weakly interacting massive particles (WIMPs) and much lighter particles called axions. Other ideas include sterile neutrinos and primordial black holes formed in the early universe. Detectors deep underground, such as LZ in the United States and XENONnT in Italy, wait for a dark-matter particle to bump into an atomic nucleus. Others search for axions turning into microwaves in strong magnetic fields. As of 2025, none has made a confirmed detection, though each result narrows down what dark matter can be.

Or is gravity different?

A minority of researchers argue that gravity behaves differently at very low accelerations, an idea called Modified Newtonian Dynamics (MOND). It reproduces many galaxy rotation curves well, but struggles with galaxy clusters, the Bullet Cluster and the cosmic microwave background. Most cosmologists consider a dark-matter particle the better explanation, while still testing alternatives.

Mapping the invisible: Euclid and beyond

ESA's Euclid space telescope launched on 1 July 2023 to map billions of galaxies across about a third of the sky. By measuring the subtle lensing distortions in their shapes, it is building a three-dimensional map of how dark matter is spread through the universe and how that has changed over the past 10 billion years. Its first survey data were released in 2025.

Euclid works alongside the Hubble and James Webb space telescopes, ground-based surveys and the Vera C. Rubin Observatory, which began observing in 2025. Together they test whether dark matter clumps exactly as the leading 'cold dark matter' model predicts, including in the smallest dwarf galaxies, where some long-standing puzzles remain.

Surprising facts about Dark Matter

If the leading particle theories are right, vast numbers of dark-matter particles stream through your body every second without leaving a trace.
Fritz Zwicky described the unseen mass in the Coma Cluster as 'dunkle Materie' in 1933, decades before the idea was widely accepted.
The Milky Way's dark-matter halo is thought to extend far beyond its visible disc and to contain most of the galaxy's mass.
A few galaxies, such as some ultra-diffuse galaxies, appear to contain very little dark matter, which itself is evidence that dark matter is a real substance rather than a quirk of gravity.
Dark matter clumped together first after the Big Bang, creating gravitational wells into which ordinary gas later fell to form galaxies.

Frequently asked questions

What is dark matter in simple terms?: Dark matter is matter that we cannot see because it does not give off or block light, but whose gravity we can measure. It holds galaxies and galaxy clusters together and outweighs ordinary matter by about five to one.
Has dark matter ever been detected directly?: Not as of 2025. Its gravitational effects are measured in many independent ways, but underground detectors and particle colliders have not yet caught a dark-matter particle. Each experiment rules out some possibilities and narrows the search.
What is the difference between dark matter and dark energy?: Dark matter pulls things together through gravity and clumps around galaxies. Dark energy is spread smoothly through space and drives the accelerating expansion of the universe. They make up about 27% and 68% of the universe's energy content respectively.
Who discovered dark matter?: Fritz Zwicky first proposed unseen matter in the Coma Cluster in 1933. Vera Rubin and Kent Ford's measurements of galaxy rotation in the 1970s provided the evidence that convinced most astronomers it was real.
Could dark matter be black holes?: Ordinary black holes formed from dead stars cannot account for it, because there are not enough of them. Primordial black holes formed in the early universe remain a possibility for some mass ranges, but observations have ruled out many of them as the main ingredient.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources