What is the universe? How big, how old, what's in it

The universe is everything that exists: all of space and time, and all the matter and energy in them, from atoms to galaxies. It began expanding from a hot, dense state about 13.8 billion years ago and is still expanding. The part we can see, the observable universe, is about 93 billion light-years across. Most of what's in it is dark matter and dark energy, which nobody has yet identified.

In short

• The universe has no known edge; what we can measure is the observable universe, the region light has had time to reach us from since the Big Bang.
• It is about 13.8 billion years old but about 93 billion light-years across, because space itself has stretched while the light was traveling.
• Ordinary matter, everything made of atoms, is only about 5% of it; the rest is dark matter (about 27%) and dark energy (about 68%).

How big is the universe, in steps you can picture?

Distances in space are measured in light-years: the distance light travels in a year, about 9.46 trillion kilometers. Even that unit is hard to feel, so here is a scale model. Shrink the distance from Earth to the Sun, about 150 million km, to 1 meter. There are about 63,200 Earth–Sun distances in a light-year, so in the model a light-year is 63.2 km. Everything else follows from that.

Step	Real size or distance	In the model (Earth to Sun = 1 m)
Earth to the Sun	150 million km, light takes 8.3 minutes	1 meter
Sun to Neptune, the outermost planet	30 times that	30 meters
Nearest other star, Proxima Centauri	4.2 light-years	268 km, a long day's drive
Across the Milky Way, our galaxy	about 100,000 light-years	6.3 million km, 16 times the real distance to the Moon
To the Andromeda galaxy, our big neighbor	about 2.5 million light-years	158 million km, about the real distance from Earth to the Sun
Across the observable universe	about 93 billion light-years	about 0.6 light-years, a seventh of the real distance to the nearest star

Even shrunk so that the Sun sits 1 meter away, the observable universe would stretch about 0.6 light-years, some 1,300 times farther than the real Neptune. Its scale is mostly emptiness: between the stars of a galaxy, and between galaxies, there is almost nothing.

What is the universe made of?

On the scale of things you can see, the universe is built in layers. Planets orbit stars, as ours do in the solar system. Stars gather in galaxies of millions to trillions; ours is the Milky Way, with a few hundred billion. Galaxies gather in groups and clusters, and clusters in superclusters strung along vast filaments with huge voids between them, a pattern astronomers call the cosmic web. Estimates of how many galaxies the observable universe holds run from a few hundred billion to about 2 trillion.

But all of that, every star, planet, gas cloud and person, is ordinary matter, and it is a small share of the total. NASA's figures, from its dark energy overview, are:

• Ordinary matter, about 5%. Everything made of atoms. Most of it is thin gas between galaxies, not stars.
• Dark matter, about 27%. Something that has mass and pulls with gravity but gives off no light. Galaxies spin so fast that their visible matter couldn't hold them together without it, and its gravity bends light from more distant galaxies.
• Dark energy, about 68%. Whatever is making the universe's expansion speed up. It was discovered in 1998, when two teams measuring distant exploding stars found that they were farther away than a slowing expansion would allow.

Neither dark matter nor dark energy has been detected directly. The names describe effects, not known things.

How do we know how old the universe is?

Three lines of evidence point to the same story. First, distant galaxies are moving away from us, and the farther they are, the faster they recede, as Edwin Hubble showed in 1929. Run that expansion backward and everything converges on a hot, dense beginning, the Big Bang.

Second, the whole sky glows faintly in microwaves, the cosmic microwave background. It is light released about 380,000 years after the beginning, when the universe had cooled enough for atoms to form and light to travel freely. The European Space Agency's Planck satellite mapped it in detail and, from its pattern, measured an age of 13.82 billion years in its first results in 2013; later analyses settled on about 13.8 billion.

Third, the oldest stars we can date come out a little younger than that, or about the same age within their measurement uncertainties. A universe younger than its oldest stars would mean something had gone wrong, and no well-measured star clearly breaks that rule.

If it's 13.8 billion years old, how is it 93 billion light-years across?

This is the most common puzzle, and the answer is that space itself stretches. Light that left a galaxy 13.8 billion years ago has traveled for 13.8 billion years, but the space it crossed kept expanding behind it. That galaxy is now about 46 billion light-years away. Count 46 billion light-years in every direction and you get a sphere about 93 billion light-years across.

Nothing is moving through space faster than light. The speed of light limits motion through space, not the stretching of space itself. Beyond the observable edge there is almost certainly more universe; its light simply hasn't had time to reach us, and some never will. Whether the whole universe is finite or infinite is unknown.

What don't we know yet?

A great deal, and the gaps are large. Nobody knows what dark matter is: experiments built to catch passing dark matter particles have so far found none. Nobody knows what dark energy is, or whether it stays constant over time.

Still argued

The universe's expansion rate, called the Hubble constant, comes out differently depending on how it's measured. Working forward from the cosmic microwave background gives about 67 kilometers per second for every megaparsec (3.26 million light-years) of distance. Measuring nearby galaxies directly gives about 73. In 2024, measurements with the James Webb Space Telescope by the team led by Adam Riess backed the higher value and, according to that team, ruled out a simple measurement error. A second team using Webb, led by Wendy Freedman, found about 70, close enough to the lower value that it saw no need for new physics. Some cosmologists think the gap points to new physics, perhaps dark energy that changes over time; others expect it to shrink as measurements improve. It is unresolved.

Other open questions are about what lives in it. Some regions hold black holes billions of times the Sun's mass, which formed earlier than anyone expected. And whether life exists anywhere else, despite how many planets there seem to be, is the puzzle known as the Fermi paradox.

Check yourself

• Why can the observable universe be 93 billion light-years across if it is only 13.8 billion years old?
• What are the three lines of evidence for the universe's age, and why do they agree?
• What is dark matter evidence of, if no one has detected it directly?

Sources

• Dark energy, dark matter — NASA
• Universe overview — NASA
• Planck reveals an almost perfect universe — European Space Agency, 2013
• Webb and Hubble confirm universe's expansion rate — ESA/Webb, 2024
• Status report on the Chicago-Carnegie Hubble Program — Freedman and others, arXiv, 2024
• Observable universe — Wikipedia