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 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, and subatomic particles. The Universe can even be understood to encompass all of mathematics, all concepts and ideas, and, even, all thoughts and emotions.The part of the universe that we can see, referred to as the observable universe, has the 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. Scientific observation of the Universe has led to inferences of its earlier stages, 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 that describes the early development of the universe, which is estimated to have begun 13.798 ± 0.037 billion years ago.The universe is mostly empty space, or a vacuum. Considering only ordinary matter, the universe has a density of 9.9 x 10−30 g/cm3 which is equivalent to only 6 protons in one cubic meter of emptiness. The universe is expanding, diluting the matter in it. Observations of supernovae have shown that the universe is not only expanding, but it is expanding at an accelerating rate.There are various multiverse hypotheses, in which some physicists have speculated that our Universe might be one among many, or even an infinite number, of universes that likewise exist. However, this theory is yet to be confirmed.