Editing Outer space (section)

== Formation and state ==
{{Main|Big Bang}}
[[File:CMB Timeline300 no WMAP.jpg|upright=1.5|thumb|alt=An artist's concept of the expanding universe opening up from the viewer's left, facing the viewer in a 3/4 pose.|Timeline of the [[expansion of the universe]], where space is represented schematically at each time by circular sections. On the left, the dramatic expansion of [[cosmic inflation|inflation]]; at the center, the expansion [[accelerating expansion of the universe|accelerates]] (artist's concept; neither time nor size are to scale)]]

The size of the whole universe is unknown, and it might be infinite in extent.{{sfn|Liddle|2015|pp=33}} According to the Big Bang theory, the very early universe was an extremely hot and dense state about [[age of the universe|13.8&nbsp;billion years ago]]<ref name=planck_2013 /> which rapidly [[expansion of the universe|expanded]]. About 380,000 years later the universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen—the so-called [[Recombination (cosmology)|recombination epoch]]. When this happened, matter and energy became decoupled, allowing photons to travel freely through the continually expanding space.<ref name="SciAm301_1_36"/> Matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep [[vacuum]] that forms what is now called outer space.{{sfn|Silk|2000|pp=105–308}} As light has a finite velocity, this theory constrains the size of the directly observable universe.<ref name="SciAm301_1_36"/>

The present day [[shape of the universe]] has been determined from measurements of the [[Cosmic microwave background radiation|cosmic microwave background]] using satellites like the [[Wilkinson Microwave Anisotropy Probe]]. These observations indicate that the [[spatial geometry]] of the observable universe is "[[Flatness (cosmology)|flat]]", meaning that photons on parallel paths at one point remain parallel as they travel through space to the limit of the observable universe, except for local gravity.<ref name="WMAP"/> The flat universe, combined with the measured mass density of the universe and the accelerating [[Hubble's law|expansion of the universe]], indicates that space has a non-zero [[vacuum energy]], which is called [[dark energy]].{{sfn|Sparke|Gallagher|2007|pp=329–330}}

Estimates put the average [[energy density]] of the present day universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and baryonic matter (ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters.<ref name=nasa_wmap/> The density of the universe is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and [[black hole]]s—to conditions in vast [[Void (astronomy)|voids]] that have much lower density, at least in terms of visible matter.<ref name=aj89_1461/> Unlike matter and dark matter, dark energy seems not to be concentrated in galaxies: although dark energy may account for a majority of the mass-energy in the universe, dark energy's influence is 5 [[Order of magnitude|orders of magnitude]] smaller than the influence of gravity from matter and dark matter within the Milky Way.<ref name=rvmphys_75_559 />