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. However, the term Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world or Nature.
Current interpretations of astronomical observations indicate that the age of the Universe is 13.73 ( ± 0.12) billion years, and that the diameter of the observable Universe is at least 93 billion light years, or 8.80 × 1026 metres. (It may seem paradoxical that two galaxies can be separated by 93 billion light years in only 13 billion years, since special relativity states that matter cannot be accelerated to exceed the speed of light in a localized region of space-time. However, according to general relativity, space can expand with no intrinsic limit on its rate; thus, two galaxies can separate more quickly than the speed of light if the space between them grows.) It is uncertain whether the size of the Universe is finite or infinite.