The universe is about 13.8 billion years old, and light travels at a fixed speed. So there is a limit to how far we can see: only light that has had time to reach us since the beginning can be seen at all. Everything inside that limit is the observable universe. It is a sphere with Earth at its centre, though any observer anywhere would find themselves at the centre of their own.
Bigger than its age
You might expect the edge to be 13.8 billion light-years away. It is much further: about 46.5 billion light-years, making the observable universe about 93 billion light-years across. The difference comes from the expansion of space. While the oldest light was travelling towards us, the space it crossed kept stretching, and the matter that sent it was carried further away. The distance to that matter today is far greater than the distance the light itself covered.
Galaxies near the edge are moving away from us faster than light. This does not break relativity, which limits motion through space, not the stretching of space itself.
The edge
The farthest thing we can see is not a galaxy but a glow. For its first 380 000 years or so, the universe was a hot, opaque fog of charged gas. When it cooled enough for atoms to form, light could travel freely for the first time. That light, stretched by expansion into microwaves, still arrives from every direction. It is the cosmic microwave background, and it marks the edge of what light can show us.
The most distant galaxies sit just inside it. In 2024 the James Webb Space Telescope confirmed JADES-GS-z14-0, a galaxy seen as it was less than 300 million years after the Big Bang. In 2025 a team led by Rohan Naidu reported MoM-z14 at redshift 14.44, light that set out about 280 million years after the Big Bang.
How much is in it?
Estimates of the number of galaxies vary. A 2016 study by Christopher Conselice and colleagues suggested about two trillion, counting the many small, faint galaxies of the early universe. Measurements of the faint background light from far out in the Solar System, made with the New Horizons probe in 2021, pointed to fewer: hundreds of billions. All together, the observable universe holds something like 10⁸⁰ atoms.
Beyond the edge
The observable universe is not the whole universe. Nobody knows how large the whole is, or whether it is infinite. Because expansion is speeding up, some of what we can see now will one day slip out of reach: light we send today can never reach galaxies more than about 16 billion light-years away.