The universe is everything that exists: all of space, time, matter, and energy. It began about 13.8 billion years ago in a hot, dense state and has been expanding and cooling ever since. It contains billions of galaxies, each with billions of stars, and possibly countless planets. Its size is unknown, but the observable universe is about 93 billion light-years across.

The universe is not static. It is expanding, and the expansion is accelerating. Galaxies are moving apart, and the space between them is stretching. The ultimate fate of the universe depends on dark energy, which makes up about 68 percent of its energy content. If dark energy remains constant, the universe will expand forever, growing colder and emptier. If it changes, the outcome could be different.

The universe is also evolving. It started as a nearly uniform soup of particles and radiation. Slight density fluctuations grew under gravity, forming galaxies, clusters, and superclusters. The first stars ignited a few hundred million years after the Big Bang. The first galaxies formed soon after. The universe has been changing ever since.

We are part of the universe, not separate from it. The atoms in our bodies were forged in stars. The space we occupy is expanding with everything else. The study of the universe is, in a sense, the universe studying itself.