Universe

The universe is the totality of all that exists: matter, energy, space, and time, governed by the laws of physics. What astronomers can observe is called the "observable universe," because light takes time to reach us, and so distant objects whose light has not yet arrived cannot be seen.

The most widely accepted account of the universe's origin is the Big Bang model: the universe began in an extremely hot and dense state, then expanded and cooled. This expansion is measured by the redshift of light from distant galaxies, which shows that most galaxies are moving away from us.

Structure

Observable matter is organized into stars, which form galaxies. Galaxies cluster into groups and superclusters, held together by gravity and connected by a vast network, with large voids between them. The Solar System, which contains Earth, is part of the Milky Way galaxy.

Composition

According to modern cosmological estimates, ordinary matter—made of atoms—makes up only a small fraction of the universe's total content. The majority is divided between dark matter, which emits no light but exerts gravitational pull, and dark energy, linked to the accelerating expansion of the universe. Both remain poorly understood.

Age and Size

Measurements of the cosmic microwave background and observations of expansion rates indicate the universe is approximately 13.8 billion years old. The observable universe is larger than the distance light has traveled, because space itself has expanded during that light's journey.

Research and Observation

Cosmology is studied using telescopes on Earth and in space, which observe visible light, infrared, and radio radiation. Gravitational waves are also used, along with experiments on small particles that help explain the universe's earliest states. Results are compared with mathematical models to test theories.