Universe

The universe is defined as everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them

The universe is very large and possibly infinite in volume. The observable matter is spread throughout the universe. On small length-scales, matter is observed to form "clumps", superclusters and, the largest-scale structures such as the Great Wall of galaxies.

The universe is old and evolving. The most precise estimate of the universe's age is 13.73±0.12 billion years old, based on observations of the cosmic microwave background radiation. Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then-available data and conceptions of the universe. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang; however, still more careful measurements are required to determine which theory is correct