The universe is everything that exists, including all space, time, matter, and energy. It contains billions of galaxies, and each galaxy has stars, planets, and other cosmic objects. Scientists believe the universe began around 13.8 billion years ago with the Big Bang, a massive expansion that started from a very hot and dense point. Since then, the universe has been growing larger, with galaxies moving farther apart. While we can see planets, stars, and galaxies, most of the universe is made up of dark matter and dark energy, which we cannot see but know are there because of their effects. In simple terms, the universe is the total of everything—seen and unseen—that makes up reality.