The universe is everything that exists: all the matter, energy, space, and time we know about. It includes Earth, the Sun, the billions of other stars in our galaxy, the billions of other galaxies beyond ours, and all the empty space in between. When astronomers study the universe, they're studying the biggest possible subject, because there's nothing outside it to compare it to.

Scientists believe the universe began about 13.8 billion years ago in an event called the Big Bang, when everything expanded outward from an incredibly hot, dense state. The universe has been expanding ever since, with galaxies moving away from each other like dots on an inflating balloon.

The word can also mean someone's entire world or experience. A young child's universe might center on home and family, while a teenager's universe expands to include school, friends, and new interests. When someone says “you're the center of my universe,” they mean you're the most important thing in their life. People sometimes talk about parallel universes in science fiction stories, imagining completely separate realities with their own rules and inhabitants, though scientists debate whether such things might actually exist.