The Universe

The Universe is the totality of everything that exists, has existed, and ever will exist. Similar meaning is sometimes conveyed with the words cosmos, world, reality, and nature. The Universe includes all of spacetime; the entire contents of outer space; all matter, energy, dark matter, and dark energy; all galaxies, stars, and planets; all humans and every living thing; all molecules, atoms, subatomic particles, photons, and their wave dualities; all physical constants, physical laws and fundamental interactions. The Universe can even be understood to encompass all of mathematics, all concepts and ideas, and all thoughts and emotions.

The part of the universe that we can see, referred to as the observable universe, has a diameter of about 93 billion light years (28 billion parsecs) and a volume of 4×1083 liters. The size of the whole universe is not known - it may be infinite. The universe is mostly empty space, close to a vacuum. Considering only ordinary matter, the density of the Universe is 9.9 x 10−30 g/cm3, equivalent to a mass density of 5.9 protons per cubic meter. The universe is expanding, and the rate of its expansion is increasing. Astronomical observations have led to inferences of the earlier stages of the universe, which appear to have been governed by the same physical laws and constants throughout most of its extent and history. The Big Bang theory is the prevailing cosmological model describing the early development of the universe, which is estimated to have begun 13.798 ± 0.037 billion years ago. Some physicists have speculated that there is more than one universe. (Wikipedia)