What is dark matter?

More than 80% of the universe's matter is invisible, and we don't know what it is. This mysterious substance, known as dark matter, doesn't emit or reflect light, making it impossible to see directly.

But without it, galaxies would fly apart. Here's what scientists know — or think they know — about this cosmic puzzle.

Related: If dark matter is 'invisible,' how do we know it exists?

What is dark matter?

Dark matter is an invisible, mysterious substance that makes up more than 80% of the matter in the universe — but scientists aren't sure what it is.

While it doesn't emit, absorb, or reflect light, we know dark matter exists because of the way its gravity affects visible matter. Galaxies rotate faster than they should based on the mass we can see, and large-scale structures in the universe form in patterns that only make sense if there’s much more matter out there than meets the eye.

Dark matter may hold the key to understanding how galaxies form, how the universe evolved, and even what it's made of at the deepest level.

How do we know dark matter exists?

Dark matter doesn’t interact with light, so we can’t see it directly. Instead, scientists detect its presence through its gravitational influence on visible objects.

Here are a few key lines of evidence for dark matter:

1. Galaxy rotation curves

When astronomers measure how fast stars orbit the centers of galaxies, they find something strange. The stars at the edges of galaxies are moving much faster than they should be if only visible matter were present. This suggests that something unseen — dark matter — is adding to the galaxy’s mass and keeping those stars from flying off into space.

2. Gravitational lensing

According to Einstein’s theory of general relativity, massive objects can bend the path of light. When light from distant galaxies travels toward us, it sometimes passes near invisible clumps of dark matter, which warp the light’s path in a phenomenon called gravitational lensing. By studying how light is distorted by galaxy clusters, astronomers have been able to create a map of dark matter in the universe.

3. The Cosmic Microwave Background (CMB)

The CMB is the leftover radiation from the Big Bang, and it contains tiny temperature fluctuations that reveal how matter was distributed in the early universe. The pattern of these fluctuations only makes sense if dark matter was already present, influencing how ordinary matter clumped together.

4. Large-scale structure of the universe

Computer simulations of the universe's evolution show that galaxies and galaxy clusters form the way they do only if there’s a huge amount of dark matter. Without it, structures would be much smaller and more scattered.

What is dark matter made of?

Despite decades of study, scientists still don't know exactly what dark matter is made of. They do know it's not made of ordinary atoms — the same kind of matter that makes up stars, planets, and people. That kind of matter is called "baryonic," and dark matter appears to be "non-baryonic."

Here are some of the leading candidates for what dark matter might be:

WIMPs (Weakly Interacting Massive Particles)

These hypothetical particles are among the top contenders. They would have mass and interact via gravity and possibly the weak nuclear force, but not with light or electromagnetism, which would make them invisible. Large underground detectors have been trying to catch one of these elusive particles in action, but so far, no luck.

Axions

Axions are ultra-light particles that could exist in large numbers and act like a cold, invisible fluid throughout space. They’re another strong candidate for dark matter, especially since they might help solve other physics puzzles, such as the strong-CP problem in particle physics.

Sterile neutrinos

Neutrinos are ghostly particles that barely interact with matter. Sterile neutrinos would be even more elusive than regular neutrinos, and if they exist, they could account for some or all of dark matter.

Primordial black holes

Some scientists have considered that dark matter could be made of tiny black holes formed in the early universe. These would be small and difficult to detect, but they remain a long-shot possibility.

Could it just be gravity that is wrong?

A few researchers have proposed that dark matter doesn't exist at all — instead, maybe our understanding of gravity breaks down at large scales. One of the most prominent alternative theories is called Modified Newtonian Dynamics, or MOND. It adjusts how gravity behaves at very low accelerations, like those on the outskirts of galaxies.

However, MOND and similar theories struggle to explain all of the evidence, especially the detailed patterns in the cosmic microwave background and gravitational lensing. That's why most scientists still think dark matter is a real substance — one we just haven’t been able to detect directly yet.

Dark matter Q&A with an expert

We asked Glenn Starkman, a Distinguished University Professor and co-Chair of Physics and Professor of Astronomy at Case Western Reserve University, a few frequently asked questions about gravity. 

Glenn Starkman

Distinguished University Professor and co-Chair of Physics and Professor of Astronomy at Case Western Reserve University, Director of the Institute for the Science of Origins, Director of the Center for Education and Research in Cosmology and Astrophysics.

How are scientists searching for dark matter?

Physicists and astronomers are using multiple strategies to try to detect dark matter particles or understand their nature.

Direct detection experiments

These experiments, often located deep underground to shield them from cosmic rays, are designed to catch a dark matter particle interacting with a detector. Experiments like LUX-ZEPLIN (LZ) and XENONnT are pushing sensitivity to new levels.

Particle colliders

The Large Hadron Collider (LHC) smashes particles together at high energies, potentially creating dark matter particles that escape detection but leave behind missing energy and momentum in the collision data.

Astronomical observations

Telescopes around the world and in space continue to map the distribution of dark matter using gravitational lensing and galaxy surveys. Instruments like the Vera C. Rubin Observatory and the European Space Agency's Euclid mission will provide even better data.

Axion and neutrino experiments

Dedicated experiments like ADMX (Axion Dark Matter Experiment) and searches for sterile neutrinos are testing other potential dark matter candidates.

Fast facts about dark matter

• Dark matter makes up about 27% of the universe. Ordinary matter is only 5%. The rest is dark energy.
• We can't see it directly, but its gravitational effects are everywhere.
• Galaxies rotate faster than expected, which is one of the main clues pointing to dark matter.
• WIMPs and axions are two of the top candidates for what dark matter might be.
• No direct detection yet, but experiments are ongoing around the world.

Additional resources

You can read more about dark matter on the website of the U.S. Fermi National Accelerator Laboratory (Fermilab), which runs high-energy experiments in cutting-edge particle colliders with the goal of discovering particles that would fill the gaps in our understanding of the universe. The European Organization for Nuclear Research (CERN), the largest particle physics laboratory in the world, is also on a quest to find missing dark matter particles. NASA discusses the difference between dark matter and dark energy in this article.

Bibliography

NASA, Dark Energy, Dark Matter, https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

Clegg, B. Dark Matter and Dark Energy: The Hidden 95% of the Universe, Icon Books, August, 2019

CERN, Dark Matter, https://home.cern/science/physics/dark-matter