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.

Astronomical observations indicate that the universe is 13.73 ± 0.12 billion years old and at least 93 billion light years across. According to the prevailing scientific theory, the universe has expanded from a gravitational singularity known as the Big Bang, a point in space and time at which all the matter and energy of the observable universe were concentrated. Since the Big Bang, the universe has expanded to its present form, possibly with a brief period of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating, and that most of the matter and energy in the universe is fundamentally different from that observed on Earth and not directly observable (cf. dark matter and dark energy). The imprecision of current observations has hindered predictions of the ultimate fate of the universe.

Experiments suggest that the universe has been governed by the same physical laws and constants throughout its extent and history. The dominant force at cosmological distances is gravity, and general relativity is currently the most accurate theory of gravitation. The remaining three fundamental forces and all the known particles on which they act are described by the Standard Model. The universe has at least three dimensions of space and one of time, although extremely small additional dimensions cannot be ruled out experimentally. Spacetime appears to be smoothly and simply connected, and space has very small mean curvature, so that Euclidean geometry is accurate on the average throughout the universe.

The word "universe" is usually defined as encompassing everything. However, using an alternate definition, some have speculated that this "universe" is one of many disconnected "universes", which are collectively denoted as the multiverse. For example, in bubble universe theory, there are an infinite variety of "universes", each with different physical constants. Similarly, in the many-worlds hypothesis, new "universes" are spawned with every quantum measurement. Since these universes are, by definition, disconnected from our own, these speculations cannot be tested experimentally.

Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative models were developed by the ancient Greeks, who proposed that the universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size – corresponding to the fixed stars, the Sun and various planets – rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus' heliocentric model and the Newtonian model of the solar system, respectively. Further improvements in astronomy led to the characterization of the Milky Way, and the discovery of other galaxies and the microwave background radiation; careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.