How Big Is the Universe?
Quick Answer
The universe is so big that it is almost impossible for the human brain to fully grasp. The observable universe, the part we can see, stretches about 93 billion light-years across. And there may be even more beyond what we can observe. It contains billions of galaxies, each with billions of stars and planets.
Explaining By Age Group
Ages 3-5 Simple Explanation
You know how sometimes something seems really, really far away, like the other end of a big park? Well, the universe is way, way, way bigger than any park or any city or even the whole Earth! It is the biggest thing there is, because it is everything!
The universe is the name for all of space and everything in it: all the stars, all the planets, the Sun, the Moon, and even the empty parts in between. It holds absolutely everything there is.
The universe is so big that if you could fly in a spaceship really fast, you would never, ever reach the end. You could fly forever and ever and still not get there. That is how big it is!
Nobody knows exactly how big the universe really is. Even the smartest scientists in the whole world are still trying to figure it out. It is one of the biggest mysteries ever, and it makes space really exciting to learn about!
Ages 6-8 More Detail
The universe is everything that exists: all of space, all the galaxies, all the stars, all the planets, and all the empty space in between. When we ask how big it is, the answer is mind-blowing. It is bigger than anything you have ever tried to imagine.
Scientists talk about the observable universe, which is the part of the universe we can actually see with our best telescopes. The observable universe is about 93 billion light-years across. One light-year is the distance light travels in a whole year, about 5.9 trillion miles. So multiply that by 93 billion, and you get a number so big that it does not really mean anything to our brains anymore.
Inside the observable universe, there are about two trillion galaxies. Each galaxy has billions of stars, and many of those stars have planets. The number of planets in the universe could be in the trillions of trillions. We are talking about numbers that make your head spin.
Here is something even wilder. The observable universe might just be a small piece of the whole universe. There could be vast amounts of space beyond what we can see, stretching out in every direction. We cannot see it because light from those distant places has not had time to reach us yet since the universe began.
It can feel a little strange to think about how tiny we are compared to the universe. Earth is a small planet around an average star in a regular galaxy. But that is also what makes us special. Out of all that vastness, this little blue planet is the only place we know of where life exists.
Ages 9-12 Full Explanation
The universe is the sum total of all space, time, matter, and energy that exists. When scientists talk about how big it is, they usually mean the observable universe, which is the region of space from which light has had time to reach us since the Big Bang about 13.8 billion years ago. The observable universe has a diameter of about 93 billion light-years. Yes, that is bigger than 13.8 billion light-years because space itself has been expanding the entire time.
To put those distances in perspective, let us work our way up. Earth to the Moon is about 239,000 miles, which takes light about 1.3 seconds to cross. Earth to the Sun is about 93 million miles, or about 8 minutes at the speed of light. Our solar system to the nearest star is about 4.2 light-years. The diameter of the Milky Way galaxy is about 100,000 light-years. The nearest large galaxy, Andromeda, is about 2.5 million light-years away. And the observable universe is 93 billion light-years across. Each step in that chain is a massive jump in scale.
The observable universe contains an estimated two trillion galaxies, each holding anywhere from millions to hundreds of billions of stars. The total number of stars in the observable universe is estimated at around 200 billion trillion, which is written as a 2 followed by 23 zeros. There are more stars in the observable universe than grains of sand on every beach on Earth.
One of the most staggering facts about the universe is that it is still expanding, and that expansion is speeding up. Distant galaxies are moving away from us, and the farther away they are, the faster they are receding. Some galaxies at the edge of the observable universe are moving away from us faster than the speed of light. This does not break any laws of physics because it is space itself that is stretching, not the galaxies moving through space.
The observable universe is almost certainly not the entire universe. There is likely far more beyond the horizon of what we can see, possibly infinitely more. We just cannot observe it because its light has not reached us yet. Some theories suggest the total universe could be trillions of times larger than what we can observe, or even infinite in size. We may never know for certain.
Thinking about these scales can feel overwhelming, but it also puts things in a powerful perspective. Every problem, every worry, every conflict on Earth is happening on one tiny speck in an incomprehensibly vast cosmos. At the same time, the fact that atoms on this little speck arranged themselves into beings that can contemplate the universe is pretty remarkable in itself.
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Tips for Parents
How big is the universe can be a challenging topic to discuss with your child. Here are some practical tips to help guide the conversation:
DO: Follow your child's lead. Let them ask questions at their own pace rather than overwhelming them with information they haven't asked for yet. If they seem satisfied with a simple answer, that's okay — they'll come back with more questions when they're ready.
DO: Use honest, age-appropriate language. You don't need to share every detail, but avoid making up stories or deflecting. Kids can sense when you're being evasive, and honesty builds trust.
DO: Validate their feelings. Whatever emotion your child has in response to learning about how big is the universe, acknowledge it. Say things like 'It makes sense that you'd feel that way' or 'That's a really good question.'
DON'T: Don't dismiss their curiosity. Responses like 'You're too young for that' or 'Don't worry about it' can make children feel like their questions are wrong or shameful. If you're not ready to answer, say 'That's an important question. Let me think about the best way to explain it, and we'll talk about it tonight.'
DO: Create an ongoing dialogue. One conversation usually isn't enough. Let your child know that they can always come back to you with more questions about how big is the universe. This makes them more likely to come to you rather than seeking potentially unreliable sources.
Common Follow-Up Questions Kids Ask
After discussing how big is the universe, your child might also ask:
Is the universe infinite?
Scientists are not sure. The observable universe is finite, with a diameter of about 93 billion light-years. But the total universe beyond what we can observe might be much, much larger, or possibly infinite. Current measurements suggest the universe is flat in its geometry, which would be consistent with it being infinite, but we cannot say for certain.
What is the universe expanding into?
This is a mind-bending concept. The universe is not expanding into anything. Space itself is stretching. It is not like a balloon expanding into a room. The universe is everything, including the room. There is no outside for it to expand into. The distances between galaxies are simply getting larger as space itself grows.
How old is the universe?
The universe is approximately 13.8 billion years old. Scientists figured this out by studying the oldest light in the universe, called the cosmic microwave background, and by measuring how fast the universe is expanding and working backward to the point where everything started.
What existed before the universe?
This is one of the deepest questions in science and philosophy. The Big Bang theory describes how the universe began expanding from an incredibly hot, dense state about 13.8 billion years ago, but it does not explain what, if anything, came before. Some scientists think the question itself might not make sense because time as we know it began with the Big Bang.
Can we travel to other galaxies?
Not with any technology we have or can currently imagine. Even the closest large galaxy, Andromeda, is 2.5 million light-years away. At the speed of our fastest spacecraft, it would take billions of years to reach. For now, we explore other galaxies using light collected by telescopes.
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