Dark matter

In astronomy and cosmology, dark matter is an invisible and hypothetical form of matter that does not interact with electromagnetic radiation, including light. Scientists know it exists because of the way its gravity affects the movement of stars and galaxies. Without dark matter, the universe would look very different, and galaxies would not have formed the way they did.

Dark matter makes up a large part of the universe's mass. In the standard model of cosmology, called the Lambda-CDM model, dark matter is about 85% of all the mass in the universe, while ordinary matter makes up only 5%. Even though it is so abundant, dark matter is very hard to detect because it does not interact with light or ordinary matter except through gravity.

Scientists think dark matter might be made of tiny, undiscovered subatomic particles, such as weakly interacting massive particles or axions. Another idea is that dark matter could be made of very small primordial black holes. Studying dark matter helps us understand how the universe began and how it continues to change over time.

History

1884 to 1940

The idea of dark matter began with discussions about how many stars might exist around our Sun. Early scientists thought that many stars we can't see might be out there, hidden in the dark. Later, astronomers noticed that stars and galaxies moved in ways that couldn't be explained by what we could see. They wondered if there was more matter we couldn't see, pulling on these stars and galaxies with its gravity.

In the 1930s, a scientist studied groups of galaxies and found they had much more mass than what we could see. He thought this unseen mass, which he called "dark matter," helped hold these groups together. However, his calculations were not completely accurate by today's standards.

1970s

In the 1970s, more observations showed that galaxies were surrounded by invisible matter. Astronomers measured how stars in galaxies spin and found they moved too fast to be held together by the visible matter alone. This suggested there was a lot of unseen matter in halos around galaxies.

1980s and 90s

More observations in the 1980s and 1990s supported the idea of dark matter. These included how light from distant objects was bent by the gravity of galaxy clusters, the heat from hot gas in galaxies and clusters, and patterns in the oldest light in the universe.

2000s to present

Since the year 2000, scientists have searched for tiny particles that might make up dark matter. They built very sensitive tools to look for these particles, but haven't found any yet. This has led some scientists to think about other possibilities, like small black holes formed long ago, as a source of dark matter. The search continues to be a big mystery in science.

Technical definition

See also: Friedmann equations

In space science, we talk about different kinds of stuff in the universe. One kind is called "matter," which includes anything that spreads out as the universe grows. This is different from "radiation," which gets weaker much faster as space expands. There's also something called the "cosmological constant," which stays the same no matter how the universe changes.

"Dark matter" is a special kind of matter that we can't see with telescopes or other tools. Even though we can't see it, we know it's there because of how things move in space. Scientists use special math to describe how dark matter behaves, and it follows the same rules as other matter in the universe.

Observational evidence

Galaxy rotation curves

Main article: Galaxy rotation curve

The arms of spiral galaxies spin around their center. The amount of visible material in a spiral galaxy gets smaller as you move from the middle to the edges. If only visible material was present, the galaxy would act like a point in the center with smaller objects moving around it, much like the Solar System. According to Kepler's Third Law, we would expect the speed of these objects to slow down as they move farther from the center, just like in our Solar System. However, this is not what we see. Instead, the speeds stay the same or even increase as objects move farther from the center.

If Kepler's laws are correct, this means there must be more material in the galaxy than we can see. This hidden material is called dark matter.

Velocity dispersions

Main article: Velocity dispersion

Stars in groups must follow certain rules that help us measure how much material is in the group. By looking at how fast stars are moving, we can figure out how much material is there. For some groups of stars, the speeds do not match what we expect based on what we can see. This suggests there is more material we cannot see, which we call dark matter.

Galaxy clustering

Galaxy clusters are important for studying dark matter because we can measure their mass in three different ways:

• By looking at how fast galaxies are moving within the cluster
• By studying X-rays from hot gas in the cluster
• By using gravitational lensing to see how the cluster bends light from objects behind it

These methods usually agree that dark matter makes up about five times more mass than the visible matter.

On larger scales, maps of galaxy positions can show patterns that help us understand the universe's structure. These patterns match what we expect if there is dark matter.

Bullet Cluster

Main article: Bullet Cluster

The Bullet Cluster is formed when two clusters of galaxies collide. It is special because the place where we see the most mass (using gravitational lensing) is not where the visible material is. This is hard to explain if there were no dark matter, but easy to explain if dark matter exists. In this case, the visible gas slows down, but the dark matter does not, so we see them in different places.

Gravitational lensing

One effect of general relativity is gravitational lensing. This means that big objects can bend light from things behind them. The more mass an object has, the more it bends the light. For example, a cluster of galaxies can bend light from a faraway object like a quasar. By measuring how much the light is bent, we can figure out how much mass is in the cluster. This helps us discover dark matter.

Type Ia supernova distance measurements

Main articles: Type Ia supernova and Shape of the universe

Type Ia supernovae are used to measure distances to faraway objects. These measurements show that the universe is expanding faster over time. This expansion is often explained by something called dark energy. Since the universe seems to be flat, the total amount of everything in it should add up to a certain amount. We know how much normal matter and energy there are, but there is still a missing piece that acts like matter — this is dark matter.

Lyman-alpha forest

Main article: Lyman-alpha forest

When we look at light from faraway galaxies and quasars, we see patterns called the Lyman-alpha forest. These patterns can help us test ideas about the universe. The patterns we see agree with ideas that include dark matter.

Cosmic microwave background

Main article: Cosmic microwave background

Dark matter and normal matter act differently, even though they are both matter. In the early universe, normal matter interacted a lot with light, but dark matter did not. This means they left different signs in the cosmic microwave background, the faint light left over from the early universe. By studying these signs, we can learn about dark matter.

The cosmic microwave background has tiny temperature changes. These changes show a pattern that matches what we expect if dark matter exists. This gives strong support to the idea of dark matter.

Structure formation

Main article: Structure formation

Structure formation is the time after the Big Bang when small changes grew to form stars, galaxies, and clusters. Before this time, the universe looked the same everywhere. Small changes slowly grew bigger and created the structures we see today. Normal matter was affected by light in the early universe, which washed out these changes. But dark matter was not affected by light, so its changes could grow bigger first. This helped normal matter form structures faster.

Sky surveys and baryon acoustic oscillations

Main article: Baryon acoustic oscillations

Baryon acoustic oscillations are small changes in how much normal matter there is in the universe over large distances. These changes happened in the early universe and leave a mark we can see today. They create a preferred distance between galaxies. This effect was predicted years ago and found in big maps of galaxies. By combining these maps with other observations, we can learn more about the universe and dark matter. The results support the idea that dark matter exists.

Theoretical classifications

Main articles: Cold dark matter, Warm dark matter, and Hot dark matter

Scientists think dark matter can be sorted into three groups: cold, warm, and hot. These names describe how fast the dark matter particles moved in the very early universe, not their actual temperature. Fast-moving particles traveled farther than slow-moving ones before the universe expanded and slowed them down.

This distance they traveled is called the free streaming length. It helps scientists understand how structures like galaxies formed. Fast particles, called hot dark matter, could travel far enough to spread out density fluctuations, meaning the first large structures would be huge and then break into galaxies. Slow particles, called cold dark matter, didn’t spread out as much, so smaller structures like galaxies could form first. Observations show that galaxies formed before larger clusters, suggesting most dark matter is cold. This is why particles like neutrinos, which move very fast, cannot make up most of the dark matter.

Composition

The identity of dark matter is unknown, but there are many ideas about what it could be, as shown in the table below.

Baryonic matter

Dark matter can refer to any substance that mainly interacts with visible matter (like stars and planets) through gravity. This means it could be made of normal matter, such as protons or neutrons, which includes objects like planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes.

However, most evidence suggests that the majority of dark matter is not this type of matter. If there were enough normal matter, it would be visible when stars shine behind it. Also, theories about the beginning of the universe predict the amounts of different elements we see today, and having more normal matter would change those amounts. Searches for certain objects called MACHOs have also shown that only a small amount of dark matter could be in these forms.

Non-baryonic matter

There are two main ideas for what non-baryonic dark matter could be: new particles or primordial black holes. Unlike normal matter, these particles wouldn't help form elements in the early universe, and we only notice them through their gravitational effects. Some dark matter candidates can interact with each other or with normal particles, possibly creating observable results like gamma rays.

Particle candidates

Weakly Interacting Massive Particles

There is no exact definition of a Weakly Interacting Massive Particle (WIMP), but generally, it is a tiny particle that interacts mainly through gravity and possibly other very weak forces. Many WIMP candidates are thought to have been created in the early universe, similar to particles we know today. Having the right amount of dark matter today needs a specific self-destruction rate, which matches what we might expect for a new particle with a mass around 100 GeV.

Because some theories predict a new particle with these properties, this has been called the "WIMP miracle." Experiments try to find WIMPs by looking for their destruction products, like gamma rays, or by trying to detect them colliding with atoms in labs.

Axions

Axions are tiny particles first suggested in 1978 to solve a problem in quantum physics. They are very light and interact very weakly, making them good candidates for dark matter. If they have a mass above a certain level, they could explain dark matter and also solve this physics problem.

Particle aggregation and dense dark matter objects

If dark matter is made of weakly interacting particles, one question is whether they can form objects like planets, stars, or black holes. Historically, it was thought they could not, because they lack ways to lose energy and interact in diverse ways needed to form structures.

Primordial black holes

Primordial black holes are hypothetical black holes that formed soon after the Big Bang. In the early universe, very dense areas of subatomic matter may have collapsed to form black holes without needing the explosions that usually create stellar black holes. These black holes are not made of normal matter and could have a wide range of masses.

Interest in primordial black holes increased after the 2015 discovery of gravitational waves from black holes of about 30 solar masses, which are hard to explain with normal stellar processes. More recent observations from telescopes like the James Webb Space Telescope have also suggested the presence of very massive black holes very early in the universe, which could support the idea of primordial black holes as a part of dark matter.

Some observations have ruled out primordial black holes making up all dark matter in certain mass ranges, but recent studies suggest that if they have a broad range of masses, they could still explain dark matter or work together with other candidates.

Fine tuning issues

One challenge for the idea of primordial black holes is explaining how they formed in the early universe. Standard models of the universe's expansion create density changes that are too small to form these black holes without special conditions. Some theories suggest that natural processes in the early universe could create the needed conditions without careful adjustments.

Particle searches

If dark matter is made of tiny particles, millions or even billions of them might pass through every part of the Earth every second. Scientists are trying many ways to find these particles. Some experiments look for particles called WIMPs, while others search for particles called axions using special tools like the Axion Dark Matter Experiment. These experiments can be split into two groups: those that look for dark matter particles bumping into normal matter, and those that look for signals from dark matter particles breaking apart or disappearing into space.

Direct particle detection

Experiments for direct detection try to spot dark matter particles as they pass through Earth and hit normal matter. For WIMPs, scientists look for tiny pushes on atoms, which can create light, electricity, or heat. For axions, experiments search for turning axions into light using strong magnets. These experiments need to be deep underground to avoid interference from space rays. Big labs around the world, such as SNOLAB in Canada and LNGS in Italy, host these experiments.

WIMPs

WIMP searches often use very cold detectors or liquid detectors filled with substances like xenon or argon. Cold detectors, kept colder than minus 270 degrees Celsius, feel the heat when a particle hits an atom. Liquid detectors watch for light and electricity made when particles crash into the liquid. Big experiments like LZ in the USA and XENONnT in Italy are using these methods. As of late 2025, no one has found dark matter this way, but these experiments have learned a lot about how often WIMPs might bump into normal matter.

Axions

Because finding WIMPs has been hard, more scientists are looking for axions. Experiments like the Axion Dark Matter Experiment use special boxes to find axions. By the early 2020s, they had become very good at spotting certain kinds of axions.

Annual modulation and directionality

Even though big experiments haven’t found dark matter, some older experiments report seeing changes in their signals through the year, which they think might be caused by Earth moving through a cloud of dark matter. Other experiments try to find dark matter by looking in the direction the Solar System is moving, using special tools to spot signals that match this direction.

Indirect particle detection

Indirect detection looks for signals made when dark matter particles break apart or disappear in space. For example, in places with a lot of dark matter, like the center of the Milky Way, dark matter particles might crash together and make light or other particles. Scientists use telescopes to look for these signals. It’s tricky because other space objects can make similar signals, so scientists need to find many signals to be sure.

Some dark matter particles might pass through the Sun or Earth and get stuck, increasing the chance they will crash together and make signals like high-energy particles called neutrinos. Telescopes like IceCube look for these signals.

Astrophysical observations

Scientists also study stars and black holes to learn about dark matter. For example, if dark matter gets stuck in stars, it could keep them warmer than expected. By studying very old, cool stars, scientists can learn how often dark matter bumps into normal matter. Scientists also study how dark matter might affect the spin of black holes.

Collider searches

Another way to find dark matter is to make it in labs using big machines like the Large Hadron Collider. When particles crash together very hard, they might make dark matter, which would escape the machine without being seen. Scientists look for signals where a lot of energy seems to disappear. These experiments have not found dark matter yet, but they have helped scientists learn where to look next.

Alternative hypotheses

Further information: Alternatives to general relativity

Some scientists think that maybe we don’t need dark matter if we change our ideas about how gravity works. Normally, we use a theory called general relativity to understand gravity, but it has mostly been tested only around Earth and the Sun. We don’t know for sure if it works the same way over huge distances in space. Changing this theory might mean we don’t need dark matter.

There are many ideas about how gravity might work differently. Some well-known ones are modified Newtonian dynamics and tensor–vector–scalar gravity. Even though changing gravity sounds simple, it’s very hard to explain all the things we see in space without dark matter. Some tests have shown a little support for these ideas, but most scientists still think dark matter is real.

Besides dark matter, scientists have thought of many other ideas. These include tiny particles that are very hard to detect, galaxies without stars, and different kinds of radiation. Some theories even suggest that dark matter might have its own forces or that it could be made of something very different from what we usually think of. Here are a few of these ideas:

• Chameleon particle – A tiny particle that doesn’t affect matter much
• Dark galaxy – A galaxy with very few or no stars
• Dark radiation – A type of energy that might connect dark matter particles
• Density wave theory – Waves in gas that help keep galaxies together
• Exotic matter – Different kinds of special matter
• Feebly interacting particles
• Light dark matter – Very light dark matter particles
• Mirror matter – Matter that might look like normal matter but acts differently
• Neutralino – A particle made from parts of other particles
• Scalar field dark matter – A guess about how dark matter might behave
• Strongly interacting massive particle – A heavy particle that connects with others
• Weakly interacting slim particle – A light version of a heavy dark matter particle

In popular culture

Dark matter is often talked about in books and TV shows that mix real science with imaginary stories. In these stories, dark matter is sometimes given special powers that are very different from what scientists really think it is. For example, it was part of a story in an episode of The X-Files called "Soft Light", and it plays a big role in His Dark Materials by Philip Pullman. Sometimes, writers use the idea of dark matter to talk about things we can't see.