The universe is all of
space and time[a] and their contents.[10] It comprises all
of existence, any fundamental interaction, physical process
and physical constant, and therefore all forms of matter and
energy, and the structures they form, from sub-atomic
particles to entire galactic filaments. Space and time,
according to the prevailing cosmological theory of the Big
Bang, emerged together 13.787±0.020 billion years ago,[11]
and the universe has been expanding ever since. Today the
universe has expanded into an age and size that is
physically only in parts observable as the observable
universe, which is approximately 93 billion light-years in
diameter at the present day, while the spatial size, if any,
of the entire universe is unknown.[3]
Some of the earliest cosmological models of
the universe were developed by ancient Greek and Indian
philosophers and were geocentric, placing Earth at the
center.[12][13] Over the centuries, more precise
astronomical observations led Nicolaus Copernicus to develop
the heliocentric model with the Sun at the center of the
Solar System. In developing the law of universal
gravitation, Isaac Newton built upon Copernicus's work as
well as Johannes Kepler's laws of planetary motion and
observations by Tycho Brahe.
Further observational improvements led to the
realization that the Sun is one of a few hundred billion
stars in the Milky Way, which is one of a few hundred
billion galaxies in the observable universe. Many of the
stars in a galaxy have planets. At the largest scale,
galaxies are distributed uniformly and the same in all
directions, meaning that the universe has neither an edge
nor a center. At smaller scales, galaxies are distributed in
clusters and superclusters which form immense filaments and
voids in space, creating a vast foam-like structure.[14]
Discoveries in the early 20th century have suggested that
the universe had a beginning and has been expanding since
then.[15]
According to the Big Bang theory, the energy
and matter initially present have become less dense as the
universe expanded. After an initial accelerated expansion
called the inflationary epoch at around 10−32 seconds, and
the separation of the four known fundamental forces, the
universe gradually cooled and continued to expand, allowing
the first subatomic particles and simple atoms to form.
Giant clouds of hydrogen and helium were gradually drawn to
the places where matter was most dense, forming the first
galaxies, stars, and everything else seen today.
From studying the effects of gravity on both
matter and light, it has been discovered that the universe
contains much more matter than is accounted for by visible
objects; stars, galaxies, nebulas and interstellar gas. This
unseen matter is known as dark matter,[16] (dark means that
there is a wide range of strong indirect evidence that it
exists, but we have not yet detected it directly) having
come into existence alongside the rest of the physical
universe before gradually gathering into a foam-like
structure of filaments and voids and allowing other forms of
matter to form together into visible structures. The ΛCDM
model is the most widely accepted model of the universe. It
suggests that about 69.2%±1.2% of the mass and energy in the
universe is dark energy which is responsible for the
acceleration of the expansion of the universe, and about
25.8%±1.1% is dark matter.[17] Ordinary ('baryonic') matter
is therefore only 4.84%±0.1% of the physical universe.[17]
Stars, planets, and visible gas clouds only form about 6% of
the ordinary matter.[18]
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 the
universe might be one among many.[3][19][20]
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https://en.wikipedia.org/wiki/Universe
