Dark matter and dark energy are two invisible components that together make up about 95% of the universe. Dark matter is a form of matter that doesn’t emit or absorb light but exerts gravitational pull, holding galaxies together. Dark energy is a separate phenomenon working in the opposite direction, pushing the universe apart by accelerating its expansion. They share the word “dark” because neither can be seen directly, but they do fundamentally different things.
How Much of the Universe Is Invisible
Data from the European Space Agency’s Planck satellite, which mapped the oldest light in the universe, puts the breakdown like this: ordinary matter (the stuff that makes up stars, planets, and everything you can touch) accounts for just 4.9% of the universe’s total mass and energy. Dark matter makes up 26.8%. Dark energy accounts for the rest, roughly 68%.
That means for every atom of visible matter in the cosmos, there’s about five times as much dark matter and fourteen times as much dark energy. The universe you can see through a telescope is a small fraction of what’s actually out there.
What Dark Matter Does
Dark matter acts as invisible scaffolding for the universe. It clumps together under its own gravity, and those clumps pull in ordinary matter, creating the conditions for stars and galaxies to form. Without it, galaxies as we know them wouldn’t exist.
On the largest scales, dark matter forms a structure called the cosmic web. Dense knots of dark matter are connected by thinner filaments, like a three-dimensional spider web stretching across the observable universe. Galaxies trace this same pattern. A 2025 study published in Nature Astronomy, using data from the James Webb Space Telescope, confirmed that wherever you find a massive cluster of thousands of galaxies, you find an equally massive concentration of dark matter in the same place. As Durham University astrophysicist Richard Massey put it: “Dark matter and regular matter have always been in the same place. They grew up together.”
The Evidence for Dark Matter
The strongest early evidence came from watching how galaxies spin. Stars at the edges of a galaxy orbit the center, and physicists can measure how fast they’re moving. If visible matter were all there was, stars at the outer edges should orbit much more slowly than stars closer to the center, the same way distant planets in our solar system orbit the sun more slowly than inner ones. But that’s not what happens. Galaxy rotation curves are generally flat or even rising at the edges, meaning outer stars move just as fast as inner ones. The only explanation is that a large amount of unseen mass surrounds the galaxy, providing extra gravitational pull.
This effect is especially dramatic in small, gas-rich galaxies. In one well-studied dwarf galaxy called DDO 154, about 90% of the total mass is dark matter. In many dim galaxies, dark matter dominates over visible matter at nearly every distance from the center.
What Dark Energy Does
Dark energy works on a completely different scale. It isn’t concentrated around galaxies or clusters. Instead, it’s spread throughout the entire universe, and its effect is to push space itself apart at an ever-increasing rate.
Scientists had long assumed that the universe’s expansion, set in motion by the Big Bang, would gradually slow down over time as gravity pulled everything back together. The discovery that the opposite is happening, that expansion is speeding up, was one of the biggest shocks in modern physics.
How Accelerating Expansion Was Discovered
In 1998, two independent teams of astronomers were studying a specific type of stellar explosion called a Type Ia supernova. These explosions are useful because they all reach roughly the same peak brightness, making them reliable distance markers. By comparing how bright a supernova appears to how bright it should be, you can calculate how far away it is.
The teams, led by Adam Riess, Saul Perlmutter, and Brian Schmidt, found that distant supernovae were dimmer than expected. That meant they were farther away than their speed of recession suggested. The only way to explain this was that the expansion of the universe had been accelerating for about the last nine billion years. The three astronomers won the 2011 Nobel Prize in Physics for the discovery. In the years since, additional observations of even more distant supernovae and other cosmic phenomena have continued to confirm the finding.
Leading Theories About What They Are
For dark matter, the two most popular theoretical candidates are WIMPs and axions. WIMPs (weakly interacting massive particles) would be relatively heavy particles that interact with ordinary matter only through gravity and the weak nuclear force, making them nearly impossible to detect. Axions would be extraordinarily light particles, originally proposed to solve an unrelated problem in particle physics. Other candidates include primordial black holes, sterile neutrinos, and more exotic possibilities, but WIMPs and axions have received the most attention because they were predicted independently before dark matter became a pressing puzzle.
For dark energy, the simplest explanation is the cosmological constant, a term Albert Einstein originally added to his equations of general relativity. In this model, dark energy is a fixed property of space itself: every cubic meter of empty space contains the same amount of energy, and as space expands, more of it appears, driving further acceleration. An alternative idea called quintessence proposes that dark energy changes over time rather than staying constant. Time-varying models can address some of the awkward fine-tuning problems that the cosmological constant raises, particularly the question of why dark energy and matter happen to be at comparable densities right now, but no model fully resolves that puzzle.
The Search for Dark Matter Particles
Physicists have been trying to catch dark matter particles directly for decades, using increasingly sensitive detectors deep underground (to shield them from interference). The largest and most sensitive experiment currently running is LUX-ZEPLIN, or LZ, located nearly a mile underground in South Dakota. LZ uses a massive tank of liquid xenon and watches for the faint signal that would occur if a dark matter particle collided with a xenon atom.
In its latest results, based on 417 days of data collected between March 2023 and April 2025, LZ found no direct evidence of WIMP collisions. But the results set the strongest constraints yet on what low-mass WIMPs could look like, further narrowing the range of possibilities. The experiment also searches for other exotic particles, including axions and millicharged particles. Each null result doesn’t disprove dark matter’s existence. It tells physicists where not to look, pushing the search toward new mass ranges and interaction types.
The Hubble Tension
One of the most pressing puzzles in cosmology right now is a disagreement about how fast the universe is expanding. When scientists use the oldest light in the universe (the cosmic microwave background, from about 13.8 billion years ago) and plug it into the standard model of cosmology, they get an expansion rate of about 67.4 kilometers per second per megaparsec. But when they measure expansion using nearby objects like supernovae and variable stars, they consistently get a faster rate, around 73 km/s/Mpc.
Multiple independent techniques measuring the nearby universe all converge on that higher number: gravitational lensing of distant quasars gives 73.3, water masers in galaxy discs give 73.9, and infrared measurements of galaxy surfaces give 73.3. The gap between these two sets of measurements, about 8% to 9%, is too large to be a fluke. This discrepancy, known as the Hubble tension, suggests something may be missing from our understanding of dark energy, dark matter, or both. If dark energy isn’t truly constant but instead changes subtly over time, that could help explain the mismatch.
Mapping the Dark Universe
The European Space Agency’s Euclid telescope, launched in 2023, is designed specifically to investigate dark matter and dark energy on cosmic scales. It will measure the shapes and distances of galaxies stretching back 10 billion years, covering the entire period during which dark energy has been driving accelerated expansion.
Euclid uses two primary methods. The first is weak gravitational lensing: dark matter bends the path of light from distant galaxies, slightly distorting their shapes. By measuring those distortions across the sky, Euclid can map the three-dimensional distribution of dark matter. The second method tracks a pattern called baryonic acoustic oscillations, which are regular ripples in the distribution of galaxies left over from sound waves in the early universe. These ripples act as a cosmic ruler, letting scientists measure how the expansion rate has changed over time. Together, these tools should reveal whether dark energy has been constant or evolving, and whether general relativity holds up on the largest scales.