Universe

The universe (Latin: universus) is all of space and time^[a] and their contents,^[10] including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. According to this theory, space and time emerged together 13.787±0.020 billion years ago,^[11] and the universe has been expanding ever since the Big Bang. While the spatial size of the entire universe is unknown,^[3] it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day.Universe

the universe is a cosmological constant (or, in extensions to ΛCDM, other forms of dark energy, such as a scalar field) which is responsible for the current expansion of space, and about 25.8%±1.1% [2015] is dark matter.^[18] Ordinary (‘baryonic‘) matter is therefore only 4.84%±0.1% [2015] of the physical universe.^[18] Stars, planets, and visible gas clouds only form about 6% of the ordinary matter.^[19]

There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that likewise exist.^[3]^[20]^[21]