Does outer space end  or go on forever?
Curious Kids is a series for children of all ages. If you have a question youd like an expert to answer, send it to curiouskidsus@theconversation.com.  Siah, age 11, Fremont, California  Right above you is the sky  or as scientists would call it, the atmosphere. It extends about 20 miles (32 kilometers) above the Earth. Floating around the atmosphere is a mixture of molecules  tiny bits of air so small you take in billions of them every time you breathe. Above the atmosphere is space. Its called that because it has far fewer molecules, with lots of empty space between them. Scientists like me are able to explain a lot of what youd see. But there are some things we dont know yet, like whether space just goes on forever. Planets, stars and galaxies  At the beginning of your trip through space, you might recognize some of the sights. The Earth is part of a group of planets that all orbit the Sun  with some orbiting asteroids and comets mixed in, too. A diagram of the solar system, showing the sun and its orbiting planets. You might know that the Sun is actually just an average star, and looks bigger and brighter than the other stars only because it is closer. To get to the next nearest star, you would have to travel through a billion miles of space. If you could ride on the fastest space probe NASA has ever made, it would still take you thousands of years to get there. If stars are like houses, then galaxies are like cities full of houses. Scientists estimate there are 100 billion stars in Earths galaxy. If you could zoom out, way beyond Earths galaxy, those 100 billion stars would blend together  the way lights of city buildings do when viewed from an airplane. Recently astronomers have learned that many or even most stars have their own orbiting planets. Some are even like Earth, so its possible they might be home to other beings also wondering whats out there. Story continues  An image showing detail of one galaxy, but visually implying there are many more. You would have to travel through 6 trillion miles of space just to reach another galaxy. Most of that space is almost completely empty, with only some stray molecules and tiny mysterious invisible particles scientists call dark matter.  Using big telescopes, astronomers see millions of galaxies out there  and they just keep going, in every direction. If you could watch for long enough, over millions of years, it would look like new space is gradually being added between all the galaxies. You can visualize this by imagining tiny dots on a deflated balloon and then thinking about blowing it up. The dots would keep moving farther apart, just like the galaxies are. These are questions scientists dont have definite answers to yet. Many think its likely you would just keep passing galaxies in every direction, forever. In that case, the universe would be infinite, with no end. Some scientists think its possible the universe might eventually wrap back around on itself  so if you could just keep going out, you would someday come back around to where you started, from the other direction. One way to think about this is to picture a globe, and imagine that you are a creature that can move only on the surface. If you start walking any direction, east for example, and just keep going, eventually you would come back to where you began. If this were the case for the universe, it would mean it is not infinitely big  although it would still be bigger than you can imagine. In either case, you could never get to the end of the universe or space. Scientists now consider it unlikely the universe has an end  a region where the galaxies stop or where there would be a barrier of some kind marking the end of space. But nobody knows for sure. How to answer this question will need to be figured out by a future scientist. Hello, curious kids! Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live. And since curiosity has no age limit  adults, let us know what youre wondering, too. We wont be able to answer every question, but we will do our best. This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Jack Singal, University of Richmond. Read more:    Jack Singal does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.