The universe is everything that exists: all matter, energy, space, and time, taken together as a single system. That includes every star, planet, galaxy, and photon of light, along with the vast stretches of empty space between them. The observable universe spans about 94 billion light-years across, but the full universe likely extends far beyond what we can see.
What the Universe Is Made Of
The surprising thing about the universe is that the stuff you can see, stars, planets, gas clouds, people, makes up only about 5% of its total energy budget. The rest is invisible. Roughly 27% is dark matter, a substance that has gravitational pull but doesn’t interact with light, making it impossible to observe directly. The remaining 68% is dark energy, an even more mysterious force that appears to be driving the universe to expand faster over time.
Ordinary matter, that familiar 5%, is made of atoms built from protons, neutrons, and electrons. These atoms form hydrogen, helium, and heavier elements forged inside stars. Every object you’ve ever touched, every planet you’ve seen in a photograph, every galaxy captured by a telescope belongs to this small slice of the cosmic pie.
How It Began
The prevailing scientific model traces the universe back roughly 13.8 billion years to an event called the Big Bang. This wasn’t an explosion in space. It was the rapid expansion of space itself from an extraordinarily hot, dense state. In the earliest moments, the universe was so energetic that matter as we know it couldn’t form. Energy was simply woven into the fabric of space-time.
As expansion continued, the universe cooled. Energy converted into matter and light. Within the first few minutes, the lightest elements (hydrogen and helium) formed. For roughly 380,000 years after that, the universe was an opaque fog of charged particles. Then it cooled enough for atoms to form and light to travel freely for the first time. That ancient light still fills the cosmos today as the cosmic microwave background, a faint glow with a temperature of about 2.7 Kelvin, just barely above absolute zero. It’s the oldest signal we can detect, and it reveals that the early universe was remarkably uniform in every direction.
How the Universe Is Structured
Zoom out far enough and the universe has a pattern. Galaxies aren’t scattered randomly. They’re arranged in a vast web-like structure sometimes called the cosmic web. Long filaments and sheets of galaxies twist through space, separated by enormous empty regions called voids. These voids can stretch hundreds of millions of light-years across and are thought to fill about 95% of space. One of the largest known structures, the Sloan Great Wall, is a chain of galaxies nearly 1.5 billion light-years long, located about one billion light-years from Earth.
This structure grew from tiny density fluctuations in the early universe. Regions with slightly more matter attracted even more through gravity, eventually building up into the stars, galaxies, and galaxy clusters we observe today. The cosmic microwave background confirms this: the early universe was almost perfectly smooth, with only minuscule variations that seeded all the complexity we see now.
The Observable Universe vs. the Whole Thing
There’s an important distinction between the observable universe and the universe as a whole. The observable universe is the sphere of space from which light has had time to reach us since the Big Bang. Its edge sits about 47 billion light-years away in every direction, giving it a total diameter of about 94 billion light-years. That number is larger than 13.8 billion light-years because space itself has been expanding while light traveled toward us.
Beyond that boundary, more universe almost certainly exists. We just can’t see it yet because its light hasn’t arrived. Most cosmological models suggest the full universe is vastly larger than what we observe, and it may even be infinite. There’s currently no way to measure what lies beyond our observational horizon.
The Universe Is Expanding
Space between galaxies is stretching. This was first observed in the 1920s and has been confirmed with increasing precision ever since. The rate of this expansion is described by a value called the Hubble constant. Two different methods of measuring it, however, give slightly different answers: observations of the ancient cosmic microwave background suggest a rate of about 67 to 68 kilometers per second per megaparsec, while telescope measurements of relatively nearby supernovae and galaxies consistently produce values around 72 to 73. The James Webb Space Telescope’s largest survey of universe expansion confirmed this discrepancy, finding a value of 72.6 km/s/Mpc. This gap, known as the Hubble tension, is one of the biggest open questions in cosmology and may point to unknown physics.
In practical terms, the expansion means that distant galaxies are moving away from us, and the farther away they are, the faster they recede. This doesn’t mean Earth is at the center. Every point in the universe sees the same thing. It’s the fabric of space itself that’s stretching.
How the Universe Will End
The most widely accepted scenario for the universe’s far future is called the Big Freeze. Because dark energy is accelerating the expansion of space, galaxies will drift apart over time until they’re no longer visible to one another. In a couple of trillion years, the universe will have expanded so much that no distant galaxies will be visible from our own galaxy, which will have long since merged with its nearest neighbors.
About 100 trillion years from now, all star formation will stop. The raw materials for new stars will simply run out. After that, in what physicists call the Degenerate Era, galaxies themselves will dissolve. Stellar remnants will decay. All remaining matter will eventually be locked inside black holes. Even those won’t last forever. By roughly a googol years into the future (that’s a 1 followed by 100 zeroes), black holes will have evaporated through a process called Hawking radiation. What’s left is the Dark Era: no stars, no matter, no structure. All energy will be evenly distributed across incomprehensibly vast distances, and the temperature will hover just above absolute zero.
An alternative scenario, the Big Crunch, imagines the universe eventually collapsing back into an infinitely dense point. This was considered plausible a few decades ago, but the discovery that expansion is accelerating rather than slowing down has made it far less likely.
Could There Be More Than One Universe?
Some physicists have proposed that our universe might be one of many. Physicist Max Tegmark at MIT has categorized these ideas into four levels. The simplest version suggests that if the universe is infinite, there are regions so far away that every possible arrangement of matter occurs somewhere, including a copy of you. A second level proposes that cosmic inflation may have spawned multiple separate regions of space with different physical constants, essentially different versions of the laws of nature. A third draws on quantum mechanics, where every possible outcome of every event plays out in its own branch of reality. The fourth and most speculative suggests that every self-consistent mathematical structure corresponds to its own physical universe.
These ideas range from plausible extensions of well-tested physics to deeply speculative frameworks. None can currently be confirmed by observation. For now, the one universe we can study remains more than enough to keep cosmologists busy.