The universe consists of roughly 72% dark energy, 28% matter (most of it dark matter), and a tiny fraction of radiation. Only about 5% of the total energy budget is the ordinary matter you can see and touch: atoms, stars, planets, gas, and dust. The rest is invisible, detectable only through its gravitational effects or its influence on how space itself behaves.
The Three Main Ingredients
Modern cosmology divides the universe’s contents into three broad categories, each playing a very different role. Dark energy makes up about 72% of the total energy density. It acts like a pressure pushing space apart, driving galaxies away from each other at an accelerating rate. Nobody knows exactly what it is, but its effects are unmistakable: the expansion of the universe is speeding up, not slowing down as scientists long assumed.
Dark matter accounts for roughly 23% of the total. It doesn’t emit, absorb, or reflect light. Its presence is inferred entirely from gravity. The remaining 5% is ordinary (baryonic) matter, the stuff that makes up everything you’ve ever seen, from your own body to the most distant visible galaxy. The Planck satellite’s 2018 measurements put the total matter density at about 31.5% of the critical density, with the rest belonging to dark energy.
What Ordinary Matter Is Made Of
All ordinary matter is built from two families of elementary particles: quarks and leptons. Quarks come in six types, paired into three generations. The lightest pair, the up quark and down quark, combine in groups of three to form protons and neutrons, the building blocks of every atomic nucleus. The heavier quarks (charm, strange, top, and bottom) are unstable and exist only briefly in high-energy collisions.
Leptons also come in six types. The most familiar is the electron, which orbits atomic nuclei and makes chemistry possible. The other leptons include the muon, the tau, and three types of neutrinos. These particles interact through fundamental forces carried by another class of particles called bosons: photons carry the electromagnetic force, gluons carry the strong force that holds nuclei together, and W and Z bosons carry the weak force responsible for certain types of radioactive decay.
In practical terms, the ordinary matter in the universe is overwhelmingly hydrogen and helium. By number of atoms, the cosmos is about 92% hydrogen and 8.5% helium, with every other element (carbon, oxygen, iron, gold, and so on) making up less than one-tenth of a percent. The Big Bang itself produced only hydrogen, helium, and a trace of lithium. Every heavier element was forged later inside stars or in the violent explosions that end their lives.
The Evidence for Dark Matter
In the 1930s, astronomer Fritz Zwicky studied the Coma cluster, a collection of galaxies about 300 million light-years away. By measuring how fast individual galaxies within the cluster were moving, he calculated how much mass was needed to keep them gravitationally bound together. The answer was roughly 10 times the mass visible as starlight. Around the same time, Dutch astronomer Jan Oort reached a similar conclusion on a smaller scale by studying how stars move near the plane of our own galaxy. He estimated there must be about three times as much mass as the visible stars accounted for.
The most compelling evidence comes from galaxy rotation curves. Stars at the outer edges of a galaxy orbit the galactic center. Based on the visible mass concentrated in the core, those outer stars should be moving slowly, the same way distant planets in our solar system orbit the Sun more slowly than inner ones. Instead, their orbital speeds stay flat or even rise with distance. This means there is far more mass than what’s visible, spread out in a vast halo surrounding each galaxy. In some galaxy clusters, the ratio of total mass to visible light is as high as 300 to 1.
The leading candidates for what dark matter particles actually are include WIMPs (weakly interacting massive particles) and axions. WIMPs would interact only through gravity and the weak force, making them nearly impossible to detect directly. Axions are extremely lightweight hypothetical particles originally proposed to solve an unrelated problem in nuclear physics. Despite decades of searching, no dark matter particle has been directly detected in a lab, and the question of its true identity remains one of the biggest open problems in physics.
What Dark Energy Does
Dark energy is even more mysterious than dark matter. It doesn’t clump together, it doesn’t interact with light, and it appears to be a property of space itself. Its primary observable effect is the accelerating expansion of the universe. Galaxies are not just moving apart; they’re moving apart faster and faster over time.
One leading explanation treats dark energy as a “cosmological constant,” a fixed energy density woven into the fabric of space. As the universe expands and new space is created, more of this energy appears with it, maintaining a constant density everywhere. Another idea proposes that dark energy is a dynamic field that can vary in strength across time and space, behaving in the opposite way to normal matter and gravity. Some physicists connect it to vacuum energy, the quantum mechanical phenomenon in which pairs of particles constantly pop in and out of existence throughout empty space. But the predicted amount of vacuum energy is vastly larger than what’s observed, a mismatch so severe it’s sometimes called the worst prediction in all of physics.
How the Mix Has Changed Over Time
The universe’s composition hasn’t always looked the way it does now. One second after the Big Bang, the cosmos was a 10-billion-degree soup of light and subatomic particles. Radiation dominated the energy budget completely. Within the first five minutes, protons and neutrons fused into helium nuclei, locking in the primordial ratio of about 77% hydrogen and 23% helium by mass.
For 380,000 years, the universe was too hot for atoms to hold together. Electrons flew freely, scattering light and making the cosmos opaque. When temperatures finally dropped enough for electrons to bind with nuclei, light was released in all directions. That light still fills the universe today as the cosmic microwave background, a faint glow detectable by radio telescopes in every direction.
After that, the universe went dark for roughly 200 million years. No stars had yet formed. The cosmos was just a vast, cooling sea of hydrogen and helium gas. Gravity slowly pulled denser pockets of that gas together, eventually igniting the first stars. As the universe expanded, radiation thinned out and matter took over as the dominant component. Then, roughly 5 billion years ago, the expansion had diluted matter enough that dark energy’s influence overtook it. We now live in a dark-energy-dominated era, and that dominance will only grow stronger with time.
How It All Fits Together
On the largest scales, matter in the universe isn’t scattered randomly. It’s organized into a vast architecture called the cosmic web: threadlike filaments of galaxies, gas, and dark matter stretching across billions of light-years, with enormous empty voids between them. Galaxy groups contain up to about 100 galaxies. Galaxy clusters hold hundreds to thousands. The Coma cluster alone houses over 1,000 galaxies and stretches 20 million light-years across.
Clusters group into superclusters, and superclusters align into galactic walls, thin sheet-like structures that rank among the largest known objects. The Sloan Great Wall spans about 1.4 billion light-years. These structures trace the underlying distribution of dark matter, which acts as invisible scaffolding. Ordinary matter falls into the gravitational wells that dark matter creates, forming stars and galaxies along the filaments while the voids grow emptier over time. Dark energy, meanwhile, stretches the web itself, pushing everything further apart and ensuring that the most distant structures will eventually disappear beyond our observable horizon.