How big is the universe?

How big is the universe?

There are two answers, because there are two universes to talk about.

• The observable universe is everything close enough that its light has had time to reach us since the Big Bang. It is a sphere centred on us, about 46 billion light years in radius and 93 billion light years across.
• The whole universe is everything, including regions whose light will never reach us. Its size is unknown. Measurements show space is very close to flat, which fits a universe that is either infinite or far bigger than the part we can see.

Why 46 billion light years, when the universe is only 13.8 billion years old? Because space kept expanding while the light travelled. The most distant light we see set off 13.8 billion years ago, but the places it left are now about 46 billion light years away. Our guide to how old the universe is explains how that age is measured.

Putting the size in perspective

Distances in space span so many powers of ten that they only fit on one line as exponents. Each point below is a size in metres, written as a power of ten: each step of 1 to the right is ten times bigger.

Our own galaxy, the Milky Way, is about 100,000 light years across, according to NASA. The observable universe is nearly a million times wider. In kilometres, its radius is about 4.4 × 10²³ km.

Our address in the universe

• The Solar System sits about halfway out from the centre of the Milky Way.
• The Local Group is our small cluster of galaxies. Its biggest member besides the Milky Way is the Andromeda galaxy, about 2.5 million light years away.
• Laniakea is the supercluster we belong to. A 2014 study in Nature mapped it as about 500 million light years across, holding around 100,000 large galaxies.
• The cosmic web: on the biggest scales, superclusters link into filaments and sheets of galaxies around vast, nearly empty voids.

How far can we see?

Looking further away means looking back in time, because the light set off long ago. The most distant galaxy reported by NASA by early 2026, MoM-z14, is seen as it was about 280 million years after the Big Bang. Beyond the first galaxies, the oldest light of all is the cosmic microwave background, released about 380,000 years after the Big Bang. Nothing earlier can be seen with light, which is why the edge of the observable universe is an edge in time as much as in space.

How many galaxies are in the universe?

Astronomers estimate the number by counting galaxies in deep images of tiny patches of sky, then scaling up to the whole sky and allowing for galaxies too faint to see.

• About 2 trillion: in 2016, a team led by Christopher Conselice combined deep Hubble images with models of galaxy growth and concluded the observable universe holds at least ten times more galaxies than thought, about 2 trillion, with roughly 90% too faint and far away for today's telescopes, as NASA reported.
• Hundreds of billions: older estimates, and some NASA pages, give figures from 100 billion to a few hundred billion.
• The debate continues: in 2021, the New Horizons spacecraft measured how dark space is from beyond Pluto. It found less background light than the 2 trillion picture implied, but NASA notes this limits the total light from galaxies, not their number.

So the honest answer is somewhere between a few hundred billion and about 2 trillion. Even at the top figure, that is about 250 galaxies for every person on Earth.

How many stars are in the universe?

ESA works it out by multiplying: a typical galaxy has about 10¹¹ to 10¹² stars, and there are perhaps 10¹¹ to 10¹² galaxies. That gives roughly 10²² to 10²⁴ stars: between 10,000 billion billion and a million billion billion. The Milky Way alone has about 100 thousand million stars, ESA says.

How many planets are in the universe?

Nobody has counted, but planets turn out to be common. NASA lists more than 6,000 confirmed planets around other stars, as of October 2026. A 2012 study in Nature, using a technique called microlensing, found that stars in the Milky Way have on average at least one planet each. If that holds everywhere, the observable universe has at least as many planets as stars: 10²² or more.

How many atoms are in the universe?

About 10⁸⁰, a 1 followed by 80 zeros. You can check that from measured numbers in three steps, using values from the Planck satellite.

1. How dense is ordinary matter? Planck's measurements give an average density of ordinary matter of about 4.2 × 10⁻²⁸ kg per cubic metre. Dividing by the mass of a proton gives about 0.25 protons per cubic metre: space is incredibly empty.
2. How big is the observable universe? A sphere of radius 46 billion light years is about 4.4 × 10²⁶ m in radius, so its volume is about 3.5 × 10⁸⁰ cubic metres.
3. Multiply: 0.25 × 3.5 × 10⁸⁰ is about 9 × 10⁷⁹ protons and neutrons. Most are in hydrogen atoms, so the number of atoms is roughly 10⁸⁰.

That counts only ordinary matter. Dark matter, about five times as much by mass, isn't made of atoms at all.

Is the universe infinite?

Nobody knows. The observable universe has an edge only because light has had limited time to travel, not because space stops there. Measurements of the cosmic microwave background show that space is flat to within a small margin. A perfectly flat universe could go on forever, but it could also be finite and so large that we can't detect any curve, a little like the Earth looking flat from a field. Either way, there is no centre and no edge that anything is expanding into.

Is the universe getting bigger?

Yes. Distant galaxies are moving apart as space expands, and since the late 1990s astronomers have found that the expansion is speeding up, driven by what is called dark energy. As it continues, galaxies beyond our local group will drift out of sight over billions of years. Our Big Bang Theory course shows how the expansion was discovered. Test yourself on what the expansion does and doesn't mean, then press Mark to check it in the course.

Worked example: counting galaxies from a deep image

A deep image covers a patch of sky one ten-millionth of the whole sky and shows 10,000 galaxies. If the sky looks about the same in every direction, how many galaxies would a whole-sky survey of the same depth find? 10,000 × 10,000,000 = 10¹¹, or 100 billion. That is the count of galaxies bright enough to see. Studies like the 2016 one then estimate how many fainter ones are missing, which is where the uncertainty comes in.

Common mistakes

• Saying the universe is 13.8 billion light years across. Its observable radius is about 46 billion light years, because space expanded while light travelled.
• Treating 2 trillion galaxies as a count. It is an estimate, and others are lower.
• Saying the universe is expanding into something. Space itself is stretching. There is no outside it moves into.
• Mixing up the Milky Way and the universe. The Milky Way is one galaxy among hundreds of billions or more.
• Forgetting dark matter. Atoms make up only about a sixth of the matter in the universe.

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