universe

The universe is the totality of space and time and all forms of matter, energy, and the physical laws that govern them. It encompasses all galaxies, stars, planets, and the contents of intergalactic space, as well as the underlying spacetime geometry. The term is distinguished from the observable universe—the region of the universe that can be seen from Earth due to the finite age of the universe and the finite speed of light.

Current mainstream cosmology describes the universe as expanding since an early hot, dense state about 13.8 billion years ago (the Big Bang). The expansion is evidenced by the redshift of distant galaxies and the cosmic microwave background, a relic radiation from the early universe. The observable universe has a diameter of about 93 billion light-years.

Composition-wise, ordinary (baryonic) matter makes up about 5% of the energy density; dark matter about 27%; and dark energy about 68%. Most visible structures are organized into a cosmic web of filaments and voids, with galaxies containing stars, gas, dust, and black holes. Large-scale processes such as gravity, hydrodynamics, and radiative transfer drive structure formation.

Open questions include the nature of dark matter and dark energy, the details of cosmic inflation, the initial conditions of the universe, and whether other universes or a multiverse exist. Observational limits arise from the cosmic horizon and measurement sensitivity. Ongoing surveys and experiments aim to refine measurements of the expansion rate, matter content, and the physics of the early universe.