Dark energy and dark matter are two invisible components that together make up about 95% of the universe. According to measurements from the Planck satellite, ordinary matter (the stuff that forms stars, planets, and people) accounts for just 4.9% of the universe’s total mass and energy. Dark matter makes up 26.8%, and dark energy accounts for 68.3%. Despite dominating the cosmos, neither has been directly observed. They are inferred entirely from their effects on things we can see.
The two are fundamentally different. Dark matter pulls things together through gravity, acting as invisible scaffolding that shaped the structure of galaxies. Dark energy pushes things apart, driving the universe to expand faster and faster. Understanding both is one of the biggest open problems in physics.
What Dark Matter Actually Does
Dark matter doesn’t emit, absorb, or reflect light. You can’t see it with any telescope at any wavelength. But it has gravity, and that gravity leaves fingerprints everywhere astronomers look.
The first clue came in the 1930s, when Swiss-American astronomer Fritz Zwicky noticed that galaxies in the Coma cluster were moving so fast they should have been flung apart. Something unseen was holding them together. Then in the 1970s, American astronomer Vera Rubin found the same problem inside individual spiral galaxies. Stars at the outer edges were orbiting far too quickly to be held in place by the visible matter alone. In the decades since, observations have confirmed that galaxy rotation curves are generally flat or rising out to the farthest edges we can measure, meaning stars far from a galaxy’s center move just as fast as stars closer in. That only works if a massive halo of invisible matter surrounds each galaxy.
The effect is especially dramatic in small, dim galaxies. In some gas-rich dwarf galaxies, dark matter outweighs visible matter at virtually every distance from the center. In one well-studied case, a galaxy called DDO 154, roughly 90% of the total mass is dark.
The Cosmic Web
Dark matter didn’t just shape individual galaxies. It determined where galaxies formed in the first place. In the early universe, dark matter began clumping together under its own gravity, and those clumps then pulled in ordinary matter, creating regions dense enough for stars and galaxies to ignite. The result is a vast network called the cosmic web: dense clusters of thousands of galaxies connected by thin filaments of matter, with enormous voids in between.
Recent images from the James Webb Space Telescope have mapped this structure in unprecedented detail. Wherever astronomers see a massive galaxy cluster, they also find an equally massive concentration of dark matter in the same place. Where they see a thin strand of ordinary matter connecting two clusters, they see a matching strand of dark matter. As astrophysicist Richard Massey of Durham University put it, dark matter and regular matter “grew up together.” Dark matter provided the gravitational blueprint, and ordinary matter followed.
What Dark Matter Might Be Made Of
Nobody knows what dark matter particles actually are. The leading candidates fall into a few categories.
- WIMPs (weakly interacting massive particles) are the most widely searched-for candidate. They would be heavy and slow-moving, interacting through gravity and possibly one other force, but passing through ordinary matter almost seamlessly. When two WIMPs meet, they may annihilate each other.
- Axions are hypothetical particles at the opposite end of the scale: extremely low mass and low energy. They were originally proposed to solve a separate problem in particle physics but happen to fit the profile of dark matter.
- Primordial black holes are another possibility. These would have formed from ordinary matter in the very early universe and could range from atom-sized to larger than stars. Stephen Hawking proposed that such objects could account for the dark matter spread throughout the cosmos.
The Search for Dark Matter Particles
The most sensitive dark matter detector on Earth is the LZ experiment, buried deep underground to shield it from cosmic rays. It uses a massive tank of liquid xenon, waiting for the rare moment when a dark matter particle bumps into a xenon atom hard enough to register. Using 417 days of data collected between March 2023 and April 2025 (the largest dataset any dark matter detector has ever assembled), LZ found no sign of WIMPs in the mass range of about 3 to 9 times the mass of a proton. That’s not a failure. Each null result eliminates possibilities, narrowing down what dark matter particles could look like and how they might interact with ordinary matter.
The detector is sensitive enough that it picked up neutrinos streaming from nuclear reactions in the sun’s core, a milestone that demonstrates just how finely tuned the instrument is. LZ is scheduled to collect over 1,000 days of search data by 2028, more than doubling its current exposure and pushing its sensitivity even further.
What Dark Energy Is
Dark energy is the name for whatever is causing the universe’s expansion to speed up. It was discovered in 1998 through observations of a specific type of exploding star called a Type Ia supernova. These explosions all peak at roughly the same intrinsic brightness, which makes them useful as cosmic distance markers. By measuring how bright a distant supernova appears from Earth and comparing that to how much its light has been stretched by the expanding universe, astronomers can figure out how fast the universe was expanding at different points in its history.
What they found was startling: distant supernovae were dimmer than expected. The only explanation that fit the data was that the universe’s expansion isn’t just continuing, it’s accelerating. Something is actively pushing the universe apart, counteracting gravity on the largest scales. That something got the name dark energy.
How Dark Energy Differs From Dark Matter
The two are often mentioned together, but they do opposite things. Dark matter is gravitationally attractive. It clumps, it clusters, and it pulls galaxies together. Dark energy is gravitationally repulsive. It operates on the scale of the entire universe, pushing galaxy clusters away from each other at an increasing rate. Dark matter built the structures we see today. Dark energy is slowly pulling those structures apart.
The simplest explanation for dark energy is the cosmological constant, a fixed amount of energy woven into the fabric of space itself. In this model, every cubic meter of empty space contains a small, constant amount of energy that drives expansion. As the universe expands and more space is created, more dark energy appears with it. Other possibilities include a dynamic field called “quintessence,” where the strength of dark energy changes over time, and a more exotic option called “phantom energy,” where the repulsive force actually grows stronger.
What Dark Energy Means for the Universe’s Future
The fate of the universe depends on the precise nature of dark energy. If dark energy is a cosmological constant, the universe will keep expanding at an accelerating rate forever. Distant galaxies will gradually recede beyond our observable horizon, and the cosmos will grow colder and emptier over trillions of years. This is sometimes called the Big Freeze.
If dark energy behaves like quintessence, the expansion still accelerates but at a decreasing rate. The long-term outcome looks similar: a cold, dilute universe, but one where the repulsive force isn’t strong enough to rip apart galaxies or solar systems.
The more dramatic possibility is phantom energy, where the repulsive force grows over time and eventually becomes infinite. In this scenario, called the Big Rip, the acceleration of expansion would overcome every force in nature, including the forces holding atoms together. For a specific value of the equation’s key parameter, this could happen in roughly 20 billion years. The timeline in the final stretch would be striking: 60 million years before the end, our galaxy gets torn apart; three months before, the solar system unbinds; 30 minutes before, Earth explodes; and fractions of a second before the end, individual atoms dissociate. Current observations slightly favor the cosmological constant over phantom energy, but the measurements aren’t precise enough to rule it out.
Mapping the Dark Universe
The European Space Agency’s Euclid telescope, launched in 2023, is designed to sharpen our picture of both dark matter and dark energy. It is building a three-dimensional map of the universe by observing billions of galaxies out to 10 billion light-years, covering more than a third of the sky outside our own galaxy. By tracking how the universe has expanded and how structures have formed over the past 10 billion years, Euclid aims to pin down the properties of dark energy and dark matter with far greater precision than previous surveys. It captures images in both optical and near-infrared light and performs spectroscopy on hundreds of millions of galaxies and stars. The central questions it’s built to answer are straightforward: What is the nature of dark matter? And what is the nature of dark energy?
These are questions that touch everything from the formation of the first galaxies to the ultimate fate of the cosmos. The 95% of the universe we can’t see isn’t a minor footnote. It’s the main story, and we’re still reading the opening chapters.