What is the universe? | Live Science

The universe is literally everything, the sum of all existence. It includes all matter, like stars and galaxies. The universe also includes all radiation and all other forms of energy. No matter where or when you exist, you are a part of the universe, as is everything you experience. There is nothing outside the universe, because anything that exists is automatically included in the definition of the universe.

How old is the universe, and is it expanding?

Our best understanding of the history of the universe comes from the Big Bang theory. Observations of distant galaxies reveal that all galaxies are, on average, moving away from every other galaxy. Astronomers interpret this motion to mean that the universe itself is expanding; on the very largest scales, the distances between galaxies grow with time. This means that in the past, the universe was smaller, hotter and denser than it is today.