Dark matter and dark energy are two different things that happen to share a spooky name. Dark matter is invisible material that pulls on things with gravity and holds galaxies together. Dark energy is a property of space itself that pushes the universe apart and makes its expansion speed up. One clumps, the other spreads out evenly. One slows cosmic growth, the other accelerates it.
Together, dark matter and dark energy make up about 95 percent of the universe’s total energy and matter. According to the final results of the European Space Agency’s Planck mission, the cosmos is made of roughly 68 percent dark energy and 27 percent dark matter. Ordinary matter makes up just 5 percent. That is all the stuff in the universe we can see: stars, planets, gas, and people. Put simply, dark matter and dark energy are very different, and everything astronomers have ever seen through a telescope is a small minority.
What is dark matter?
Dark matter is matter that does not emit light, absorb it, or reflect it in any way we can detect. It does not seem to feel the electromagnetic force at all, which is why it ignores light and other electromagnetic waves. It cannot be directly seen, touched, or caught in a jar. We know it is there because of its gravitational effect. It tugs on stars, bends light, and shapes how galaxies form and move.
The word “dark” here means invisible, not black. Dark matter does exert gravity, but it is spread so thinly that a cloud of it could pass through your room right now and you would never notice. It cannot harm you or be touched. Most physicists think it is made of some kind of particle that barely interacts with ordinary matter except through gravity.
Dark matter constitutes about 85 percent of all matter in the universe, roughly five times more than ordinary (baryonic) matter. Every galaxy, including our Milky Way, sits inside a huge, roughly spherical halo of it that extends far beyond the visible stars. On the largest scales, dark matter forms a vast network of filaments called the cosmic web, and galaxies light up along its threads. Without its extra gravitational attraction, this large-scale structure would not have had time to form since the Big Bang.
What is dark energy?
Dark energy is the name for whatever is causing the expansion of the universe to speed up. It is not a substance that clumps in one place. As far as we can tell, it is spread smoothly through all of space, with the same density everywhere.
The simplest explanation is that empty space itself carries a small amount of energy, called vacuum energy. Every region of space would hold the same energy density. Albert Einstein’s equations of general relativity allow for this, in the form of the “cosmological constant,” often written with the Greek letter lambda (Λ). As the universe grows, there is more space, so there is more of this energy, and its push wins out over the pull of gravity on the largest scales.
That is why dark energy matters more today than it did in the early universe. Billions of years ago, matter was packed more tightly and gravity, an attractive force, dominated. About five billion years ago, dark energy took over, and the universe entered a phase of accelerated expansion. Astronomers have known the universe is expanding since Edwin Hubble’s work in the 1920s. The surprise was that the expansion is getting faster.
Dark matter vs dark energy: the key differences
| Feature | Dark matter | Dark energy |
|---|---|---|
| Share of the universe | About 27 percent | About 68 percent |
| What it does | Pulls things together with gravity | Pushes space apart, speeding up expansion |
| Where it is | Clumped in halos around galaxies and clusters | Spread evenly through all of space |
| Main evidence | Galaxy rotation, gravitational lensing, the Bullet Cluster, the cosmic microwave background | Distant supernovae, the cosmic microwave background, galaxy maps |
| Leading ideas | Unknown particles such as WIMPs or axions | Energy of empty space (the cosmological constant) or a changing field |
| Discovered | Hinted at in the 1930s, firmly established in the 1970s | 1998 |
| Changes with time? | Thins out as the universe expands | Stays constant, or possibly weakens slightly (still debated) |
A simple way to remember it: dark matter is the glue, dark energy is the accelerator. If you are wondering which is more powerful, dark matter or dark energy, the answer depends on scale. Inside galaxies and clusters, dark matter’s gravity wins easily. For the universe as a whole, dark energy dominates today, since it makes up more than twice as much of the total.
The evidence for dark matter
Galaxies spin too fast
In the 1970s, astronomer Vera Rubin and her colleague Kent Ford measured the velocity at which stars orbit the centers of spiral galaxies, starting with Andromeda. If a galaxy’s mass were concentrated where its light is, stars near the edge should move more slowly, the way outer planets in our solar system orbit more slowly than inner ones. Instead, the outer stars moved just as fast as the inner ones. These flat galactic rotation curves showed up in galaxy after galaxy. Under Newton’s laws of gravity, they made no sense without extra mass. Something unseen was adding mass and holding the fast-moving stars in place.
This observational evidence is one of the main reasons scientists are confident dark matter exists. Rubin was not the first to spot the problem. In 1933, Swiss astronomer Fritz Zwicky noticed that galaxies in the Coma Cluster were moving far too quickly to be held together by the visible matter alone. He called the missing mass “dunkle Materie,” German for dark matter. His work was largely overlooked for decades.
Gravitational lensing
According to general relativity, the presence of matter bends space, and bent space bends light. When light from distant galaxies passes a massive galaxy cluster on its way to us, the images get stretched into arcs or duplicated. By measuring how strongly the light is bent, astronomers can weigh the cluster. The result is always the same: clusters contain far more mass than their stars and gas can account for.
The Bullet Cluster
In 2006, astronomers using NASA’s Chandra X-ray Observatory studied the Bullet Cluster, two galaxy clusters that smashed into each other. The hot gas, which holds most of the ordinary matter, collided and slowed down in the middle. But lensing measurements showed that most of the mass had sailed straight through and sat off to the sides. The mass and the ordinary matter had been pulled apart. That is hard to explain by tweaking the laws of gravity and easy to explain if most of the mass is dark matter that barely interacts with anything.
The cosmic microwave background
The cosmic microwave background, or CMB, is the faint afterglow of the Big Bang, released about 380,000 years after the universe began. Tiny temperature ripples in this light record how matter was distributed at that time. The pattern of those ripples depends on how much ordinary matter and how much dark matter existed. Measurements from the WMAP and Planck satellites fit a universe with about five times more dark matter than ordinary matter. Computer simulations of structure formation tell the same story: without dark matter, the distribution of matter we see today in large-scale surveys could not have grown from those tiny ripples.
The evidence for dark energy
Exploding stars that were too faint
In 1998, two independent teams led by Saul Perlmutter, Brian Schmidt, and Adam Riess measured type Ia supernovae, a special kind of exploding star. These explosions all reach about the same peak brightness, so they work as “standard candles”: how bright they look tells you how far away they are.
The teams expected to find that the expansion of the universe was slowing down, pulled back by gravity. Instead, distant supernovae were fainter than predicted, which meant they were farther away than a slowing universe would allow. The universe is accelerating. The three scientists shared the 2011 Nobel Prize in Physics for the discovery.
Other lines of evidence
Since then, several independent methods have backed up the result. The CMB shows that the universe is geometrically flat, which requires a total amount of energy that ordinary and dark matter alone cannot supply. Large galaxy surveys map a pattern called baryon acoustic oscillations, frozen sound waves from the early universe that act as a cosmic ruler. Measuring that ruler at different distances tracks how the expansion rate has changed over time, and the answer again points to the need for dark energy. Since the 1990s, every independent astronomical method has agreed on this, even though nobody knows the energy source behind it.
What could dark matter be made of?
Nobody knows yet. Ordinary explanations have mostly been ruled out. Dark matter is not simply dark gas, dust, or dead stars, because the CMB and the amounts of light elements made in the early universe both limit how much ordinary matter there can be. So scientists are looking for new particles.
- WIMPs (weakly interacting massive particles) are hypothetical heavy particles that would interact through gravity and possibly a very weak force. For decades they were the favorite candidate. Giant detectors deep underground, filled with liquid xenon, wait for a WIMP to bump into an atom nucleus.
- Axions are hypothetical particles far lighter than an electron. They were first proposed to solve a different puzzle in particle physics. Experiments try to catch them turning into faint microwave signals inside strong magnetic fields.
- Other ideas include a heavier cousin of the neutrino called the sterile neutrino, and primordial black holes formed in the first moments after the Big Bang.
None of these fit into the Standard Model of particle physics, which is one reason the search is so exciting. A minority of scientists argue that there is no dark matter at all and that gravity itself behaves differently on large scales, an idea called modified Newtonian dynamics, or MOND. It explains some galaxy rotation curves well but struggles with galaxy clusters, the Bullet Cluster, and the CMB.
What could dark energy be?
Here the mystery is even deeper, and it is one of the biggest open questions in astrophysics and cosmology. The two main possibilities are:
- A cosmological constant. Space has a fixed energy density that never changes. This is the standard picture, known as Lambda-CDM (lambda plus cold dark matter). Its big problem is that quantum physics predicts a vacuum energy vastly larger than what we observe.
- A changing field. Dark energy could be a dynamic scalar field, sometimes called quintessence, whose strength slowly varies over cosmic time.
Some researchers also explore whether the laws of physics, and general relativity in particular, need to be modified on the largest scales, which would make dark energy a sign of new gravity rather than a new ingredient.
Current research: where things stand
DESI and the hint of evolving dark energy
The Dark Energy Spectroscopic Instrument (DESI), mounted on a telescope in Arizona, builds a 3D map of tens of millions of galaxies and quasars. In 2024 and again in March 2025 with its second data release, DESI reported that dark energy may be weakening over time rather than staying constant. Combined with CMB and supernova data, the preference for changing dark energy ranges from about 2.8 to 4.2 sigma, depending on which supernova sample is used.
That is intriguing but not yet a discovery. Physicists usually want 5 sigma before they claim one. DESI finished its planned five-year survey in April 2026 after observing more than 47 million galaxies and quasars, so larger analyses are still to come. Meanwhile, the final results from the Dark Energy Survey, published in early 2026, lined up most closely with the standard constant dark energy model. The question is open.
Euclid
ESA’s Euclid space telescope, launched in 2023, is mapping the shapes and positions of more than a billion galaxies. Tiny distortions in galaxy shapes caused by weak gravitational lensing reveal where dark matter sits, and the 3D distribution of galaxies tracks the history of dark energy. Euclid’s first major cosmology data release is scheduled to begin in late 2026 and continue into 2027.
The hunt for dark matter particles
The LUX-ZEPLIN (LZ) experiment, a detector holding several tons of liquid xenon about a mile underground in South Dakota, has set some of the world’s tightest limits on WIMPs. In September 2026 the team reported a single unusual event from June 2023 that known backgrounds struggle to explain. The collaboration was clear that one event is not a discovery. It is a reason to keep watching.
Other projects include the Vera C. Rubin Observatory in Chile, whose ten-year sky survey will map dark matter through lensing, and NASA’s Nancy Grace Roman Space Telescope, designed in part to study dark energy.
Why this matters
Dark energy and dark matter are not side details. They are core parts of the Big Bang theory as it is taught today, and the simple model built on them, Lambda-CDM, matches a huge range of observations. Without dark matter, galaxies like the Milky Way would probably never have formed, and without dark energy, the long-term fate of the universe would look very different. If dark energy stays constant, the universe will keep expanding faster and faster, and distant galaxies will eventually drift out of view. If it changes, that forecast changes too.
These questions also show science at work. Researchers noticed that observations did not match predictions, proposed explanations, and are now testing them with better instruments. That cycle is described in our guide to the steps of the scientific method. Light is the main tool for all of this, and if you are curious how light behaves closer to home, see why the sky is blue. For more explainers, browse our science and technology section.
Summary
- Dark matter is invisible mass that pulls with gravity and holds galaxies and clusters together. It makes up about 27 percent of the universe.
- Dark energy is a smooth property of space that makes cosmic expansion accelerate. It makes up about 68 percent.
- Ordinary matter, everything we can see, is only about 5 percent.
- Dark matter is supported by galaxy rotation curves, gravitational lensing, the Bullet Cluster, and the CMB. Dark energy was discovered through distant supernovae in 1998.
- Neither has been directly identified. DESI hints that dark energy may be changing, and Euclid, Rubin, and underground detectors like LZ are pushing for answers.
