The universe is roughly 68% dark energy, 27% dark matter, and only 5% ordinary matter. That means everything you can see, touch, or detect with a telescope makes up a tiny sliver of what actually exists. The observable universe stretches about 94 billion light-years across and contains at least two trillion galaxies, each holding billions of stars, along with gas, dust, radiation, and vast stretches of nearly empty space.
The Three Main Ingredients
Cosmologists break the universe’s total energy content into three categories. Dark energy is the dominant component at 68%, and it acts as a kind of outward pressure that is accelerating the expansion of space itself. About nine billion years after the universe began, this expansion started speeding up rather than slowing down, and dark energy is the placeholder name for whatever force is responsible. Some physicists think it could be a background energy woven into the fabric of space, sometimes called vacuum energy, which would align with a term Albert Einstein originally included in his equations of general relativity.
Dark matter makes up 27%. It doesn’t emit, absorb, or reflect light, so it’s invisible to every telescope ever built. Its existence is inferred from the way galaxies behave. Stars at the edges of galaxies orbit far faster than they should based on the visible mass alone, which means something unseen is providing extra gravitational pull. Massive galaxy clusters also bend the light of objects behind them more than their visible matter can account for. Both of these observations point to large amounts of invisible mass threaded through and around galaxies.
That leaves ordinary matter at just 5%. This is everything made of atoms: stars, planets, gas clouds, people, and every element on the periodic table. Despite being the only kind of matter we can directly observe, it’s a small minority of the universe’s total content.
Where Ordinary Matter Actually Lives
Even within that 5%, the distribution is surprising. Stars contain only about 0.5% of the universe’s total matter. Ten times more atoms are floating freely in space than are bound up in stars. And just 0.03% of all matter consists of elements heavier than hydrogen and helium, which includes carbon, oxygen, iron, and every other building block of planets and life.
Research tracking where ordinary matter resides found that 76% of it lies in the space between galaxies, in a diffuse, superheated gas called the warm-hot intergalactic medium. Another 15% sits in galaxy halos, the extended regions surrounding the visible disk of stars. Only about 9% is found within galaxies themselves, in the form of stars and cold gas. So the vast majority of “normal” matter isn’t in anything recognizable. It’s spread thinly across intergalactic space at temperatures of hundreds of thousands to millions of degrees, too hot and too sparse to form structures but still detectable through X-ray observations and certain spectral signatures.
Galaxies, Stars, and Planets
The observable universe contains at least two trillion galaxies. Earlier estimates from the Hubble Space Telescope put the number at about 200 billion, but deeper analysis revealed that roughly 90% of galaxies are too faint and too distant for current telescopes to see. Galaxies range enormously in size, from dwarf galaxies with a few billion stars to giant elliptical galaxies containing over a trillion.
Within galaxies, stars are the most familiar objects, and they come in a wide range of masses and lifetimes. The smallest red dwarfs burn for trillions of years. The largest blue supergiants exhaust their fuel in a few million years before exploding as supernovae, scattering heavy elements into space. Those heavy elements eventually clump together to form rocky planets, asteroids, and comets. Every atom of calcium in your bones and iron in your blood was forged inside a star that died before our solar system formed.
Planets are common. Observations suggest that most stars in our galaxy have at least one planet, putting the number of planets in the Milky Way alone in the hundreds of billions. Many orbit in the habitable zone of their star, where liquid water could theoretically exist on the surface.
The Cosmic Web
Zoom out far enough and individual galaxies become tiny points in a much larger pattern. Gravity pulls galaxies, galaxy groups, and galaxy clusters into twisting, threadlike structures called the cosmic web. Where these filaments intersect, matter piles up into dense knots, forming superclusters and massive walls of galaxies. Between the filaments are cosmic voids, enormous regions of nearly empty space. The overall effect looks like galaxies are resting on the surface of bubbles, with the interior of each bubble almost completely empty.
This web-like structure isn’t random. It traces the distribution of dark matter, which acts as invisible scaffolding. Ordinary matter falls into the gravitational wells that dark matter creates, so the visible galaxies we observe are essentially decorating a much larger, invisible framework.
Radiation Filling All of Space
The universe isn’t just matter and energy in the abstract. It’s also flooded with radiation left over from its earliest moments. The cosmic microwave background is a faint glow of light that fills every part of the sky, currently measured at a temperature of about 2.725 degrees above absolute zero. This radiation was released roughly 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and light to travel freely for the first time. It has been stretching and cooling ever since as space expands.
Alongside this light, the universe is also filled with relic neutrinos, ghostly particles that barely interact with anything. These were released even earlier than the microwave background, and their estimated temperature today is about 1.95 degrees above absolute zero. There are roughly 340 million of them in every cubic meter of space, passing through your body constantly without effect. Though each neutrino carries almost no mass individually, their sheer numbers mean they contribute a small but measurable fraction of the universe’s total energy.
Why the Observable Universe Has a Boundary
The observable universe is 94 billion light-years across, which seems contradictory given that the universe is only about 13.8 billion years old. The explanation is expansion. Light from the most distant visible objects has been traveling toward us for almost the entire age of the universe, but during that time, space itself has been stretching. The objects that emitted that light have since moved much farther away, putting them roughly 47 billion light-years from us in every direction.
Beyond that boundary, more universe almost certainly exists. We simply can’t see it because light from those regions hasn’t had time to reach us yet. The total universe could be vastly larger than the observable portion, or even infinite. Current measurements of the universe’s geometry suggest it is flat, which is consistent with it extending without limit, though this remains an open question.