Universe

The universe is everything that exists. It includes all things made of stuff and energy. It goes from the tiniest pieces to huge groups of stars called galaxies.

Scientists learned that the universe is always getting bigger. This has been happening for about 13.8 billion years. It all started with a huge explosion of hot material called the Big Bang. The part of the universe we can see is about 93 billion light-years across. A light-year is how far light travels in one year. We don't know how big the whole universe really is.

Long ago, people thought Earth was at the center of everything. Later, scientists discovered that the Sun is in the middle, and Earth goes around it. We found out the Sun is just one of hundreds of billions of stars. Our group of stars is called the Milky Way. There are hundreds of billions of other groups of stars like ours out there. These groups are spread out evenly everywhere in space. When you look closer, the groups of stars bunch together in clusters. These clusters form long strands with empty spaces between them, like a giant sponge.

Scientists discovered something surprising. There is much more invisible stuff in space than we can see with our eyes. We call this invisible stuff dark matter. There is also something called dark energy that makes the universe expand faster and faster. The stuff we can actually see, like stars and planets, is only a tiny part of the universe. Most of the universe is made of things we cannot see.

Scientists still wonder what will happen to the universe in the future. They also ask what came before the Big Bang. These are big mysteries that people are still trying to solve.