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The universe is the totality of space, time, matter, energy, and the physical laws that describe them. It includes all galaxies, stars, planets, and all forms of matter and energy, as well as the forces and constants that govern their interactions. The study of its origin, structure, and evolution is the focus of cosmology.

The prevailing model is the Big Bang theory, which posits that the universe began in a hot, dense state about 13.8 billion years ago and has been expanding since. Observations of cosmic expansion, the cosmic microwave background, and the large-scale distribution of galaxies support this framework. The search for understanding also involves the behavior of gravity, quantum physics, and the behavior of light across vast distances.

The observable universe is the region that can be seen from Earth given the finite speed of light and the age of the universe. It has a radius of about 46.5 billion light-years, a diameter of roughly 93 billion light-years, and contains hundreds of billions of galaxies. What lies beyond the observable region remains unknown and is a subject of speculation and ongoing research.

In terms of composition, ordinary (baryonic) matter accounts for about 5% of the universe’s energy content, while dark matter makes up about 27% and dark energy about 68%. Dark matter influences the formation of structures, and dark energy is associated with the observed acceleration of cosmic expansion. Other ideas, such as the multiverse, exist primarily in theoretical contexts and are not yet established as part of mainstream science.