Quick answer: What is dark matter? It is an invisible form of matter that does not give off, absorb or reflect light, but whose gravity shapes galaxies and the universe. It makes up about 85% of all matter and roughly 27% of the universe’s total energy content. Scientists know it exists because galaxies spin too fast, light bends too much, and cosmic structures formed too quickly to be explained by visible matter alone. What dark matter is actually made of is still unknown.
Everything we can see, from stars and planets to gas clouds and people, makes up only a small part of the universe. The rest is invisible. Dark matter is one of the biggest unsolved problems in physics, and finding out what it is could change our understanding of the cosmos.
What is the universe made of?
Measurements of the cosmic microwave background, the afterglow of the Big Bang, by the European Space Agency’s Planck satellite give this breakdown:
| Component | Share of the universe | What it does |
|---|---|---|
| Dark energy | About 68% | Drives the accelerating expansion of the universe |
| Dark matter | About 27% | Provides extra gravity that holds galaxies and clusters together |
| Ordinary matter | About 5% | Everything made of atoms: stars, planets, gas, us |
Dark matter and dark energy are different things. Dark matter pulls things together through gravity. Dark energy pushes the universe apart.
The evidence for dark matter
1. Galaxy clusters move too fast
In 1933, Swiss astronomer Fritz Zwicky studied the Coma galaxy cluster and found its galaxies moving so fast that the cluster should fly apart. He proposed that unseen “dunkle Materie” (dark matter) was holding it together.
2. Galaxies spin too fast at their edges
In the 1970s, Vera Rubin and Kent Ford measured how fast stars orbit in spiral galaxies. Stars far from the center moved just as fast as those near it. With only visible matter, outer stars should move slower, like outer planets in the solar system. The best explanation is a large halo of dark matter around each galaxy.
3. Gravitational lensing
Mass bends light. By measuring how much galaxies and clusters distort the light from objects behind them, astronomers can map where the mass is, and there is far more of it than visible matter.
4. The Bullet Cluster
In this pair of colliding galaxy clusters, the hot gas (most of the normal matter) slowed down in the crash, but lensing shows most of the mass kept moving ahead with the galaxies. That separation is hard to explain without dark matter.
5. The cosmic microwave background
The pattern of tiny temperature ripples in the Big Bang’s afterglow matches predictions only if dark matter is included in the right amount.
What could dark matter be made of?
| Candidate | What it is | Status |
|---|---|---|
| WIMPs (weakly interacting massive particles) | Heavy particles that barely interact with normal matter | Long the favorite; not detected despite very sensitive searches |
| Axions | Extremely light particles first proposed to solve a problem in particle physics | Actively searched for by experiments such as ADMX |
| Sterile neutrinos | A heavier, even shyer type of neutrino | Hypothetical |
| Primordial black holes | Black holes formed just after the Big Bang | Possible for some mass ranges; heavily constrained |
| Modified gravity (MOND and others) | Gravity behaves differently at large scales, so no dark matter is needed | Explains some galaxy data but struggles with clusters and the cosmic background |
How scientists search for dark matter
- Direct detection: Huge tanks of liquid xenon deep underground, such as LUX-ZEPLIN in South Dakota and XENONnT in Italy, wait for a dark matter particle to bump into an atom.
- Particle colliders: The Large Hadron Collider at CERN looks for “missing energy” that could signal dark matter being created.
- Indirect detection: Telescopes search for gamma rays or other signals that could come from dark matter particles colliding and annihilating.
- Sky maps: Surveys such as the Dark Energy Survey, ESA’s Euclid mission and the Vera C. Rubin Observatory map dark matter across billions of light-years using lensing.
Why dark matter matters
Without dark matter, galaxies like the Milky Way probably would not have formed when they did. In computer simulations, dark matter forms a vast “cosmic web” of filaments, and normal gas falls into these invisible structures, where it cools and forms stars. In a real sense, dark matter built the scaffolding for everything we see, including us.
What is dark matter? Key facts
- There is roughly five times more dark matter than ordinary matter.
- The Milky Way sits inside a dark matter halo much larger than its visible disk.
- Dark matter passes through ordinary matter, including Earth and your body, almost without a trace.
- No experiment has yet directly detected a dark matter particle.
Frequently asked questions
Has dark matter been proven?
Its gravitational effects are measured in many independent ways, so most astronomers are confident something is there. What it is made of has not been identified.
What is the difference between dark matter and dark energy?
Dark matter adds gravity and helps hold galaxies together. Dark energy is a mysterious effect causing the expansion of the universe to speed up.
Is dark matter the same as antimatter?
No. Antimatter is ordinary matter’s mirror twin and produces light when it meets matter. Dark matter does not interact with light at all.
Can we see dark matter?
Not directly. Scientists map it by its gravity, mainly by how it bends the light from distant galaxies.
Is dark matter related to black holes?
Mostly not. Regular black holes are far too rare to explain dark matter, although tiny primordial black holes are one possible candidate for part of it.
Related articles
Sources
- Planck Collaboration (2020). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics. doi:10.1051/0004-6361/201833910
- Rubin, V. C. & Ford, W. K. (1970). “Rotation of the Andromeda Nebula from a spectroscopic survey of emission regions.” The Astrophysical Journal. doi:10.1086/150317
- Zwicky, F. (1933). “Die Rotverschiebung von extragalaktischen Nebeln.” Helvetica Physica Acta.
- Clowe, D. et al. (2006). “A direct empirical proof of the existence of dark matter.” The Astrophysical Journal Letters.
- NASA Science: “Dark Matter”. science.nasa.gov