Universe

=== Ordinary matter ===
{{Main|Matter}}
The remaining 4.9% of the mass–energy of the universe is ordinary matter, that is, [[atom]]s, [[ion]]s, [[electron]]s and the objects they form. This matter includes [[star]]s, which produce nearly all of the light we see from galaxies, as well as interstellar gas in the [[interstellar medium|interstellar]] and [[intergalactic medium|intergalactic]] media, [[planet]]s, and all the objects from everyday life that we can bump into, touch or squeeze.<ref name="Davies2">{{cite book |author=Davies |first=P. |url=https://books.google.com/books?id=akb2FpZSGnMC&pg=PA1 |title=The New Physics: A Synthesis |date=1992 |publisher=[[Cambridge University Press]] |isbn=978-0-521-43831-5 |page=1 |language=en |access-date=May 17, 2020 |archive-url=https://web.archive.org/web/20210203103749/https://books.google.com/books?id=akb2FpZSGnMC&pg=PA1 |archive-date=February 3, 2021 |url-status=live}}</ref> The great majority of ordinary matter in the universe is unseen, since visible stars and gas inside galaxies and clusters account for less than 10 percent of the ordinary matter contribution to the mass–energy density of the universe.<ref>{{Cite journal
| last1=Persic
| first1=Massimo
| last2=Salucci
| first2=Paolo
| date=September 1, 1992
| title=The baryon content of the universe
| journal=Monthly Notices of the Royal Astronomical Society
| language=en
| volume=258
| issue=1
| pages=14P–18P
| doi=10.1093/mnras/258.1.14P
| issn=0035-8711
|arxiv=astro-ph/0502178 |bibcode=1992MNRAS.258P..14P |s2cid=17945298
}}</ref><ref>{{Cite journal |last1=Shull |first1=J. Michael |last2=Smith |first2=Britton D. |last3=Danforth |first3=Charles W. |date=November 1, 2012 |title=The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still Be Missing |url=https://iopscience.iop.org/article/10.1088/0004-637X/759/1/23 |journal=The Astrophysical Journal |volume=759 |issue=1 |pages=23 |doi=10.1088/0004-637X/759/1/23 |arxiv=1112.2706 |bibcode=2012ApJ...759...23S |s2cid=119295243 |issn=0004-637X |quote=Galaxy surveys have found ~10% of these baryons in collapsed objects such as galaxies, groups, and clusters [...]  Of the remaining 80%–90% of cosmological baryons, approximately half can be accounted for in the low-z [intergalactic medium] |access-date=February 27, 2023 |archive-date=September 21, 2023 |archive-url=https://web.archive.org/web/20230921160249/https://iopscience.iop.org/article/10.1088/0004-637X/759/1/23 |url-status=live }}</ref><ref>{{Cite journal |last1=Macquart |first1=J.-P. |last2=Prochaska |first2=J. X. |last3=McQuinn |first3=M. |last4=Bannister |first4=K. W. |last5=Bhandari |first5=S. |last6=Day |first6=C. K. |last7=Deller |first7=A. T. |last8=Ekers |first8=R. D. |last9=James |first9=C. W. |last10=Marnoch |first10=L. |last11=Osłowski |first11=S. |last12=Phillips |first12=C. |last13=Ryder |first13=S. D. |last14=Scott |first14=D. R. |last15=Shannon |first15=R. M. |date=May 28, 2020 |title=A census of baryons in the Universe from localized fast radio bursts |url=http://www.nature.com/articles/s41586-020-2300-2 |journal=Nature |language=en |volume=581 |issue=7809 |pages=391–395 |doi=10.1038/s41586-020-2300-2 |pmid=32461651 |arxiv=2005.13161 |bibcode=2020Natur.581..391M |s2cid=256821489 |issn=0028-0836 |access-date=February 27, 2023 |archive-date=November 5, 2023 |archive-url=https://web.archive.org/web/20231105012727/https://www.nature.com/articles/s41586-020-2300-2 |url-status=live }}</ref>

Ordinary matter commonly exists in four [[state of matter|states]] (or [[phase (matter)|phases]]): [[solid]], [[liquid]], [[gas]], and [[plasma (physics)|plasma]].<ref>{{cite book |url=https://openstax.org/books/chemistry-2e/pages/1-2-phases-and-classification-of-matter |title=Chemistry 2e |publisher=OpenStax |first1=Paul |last1=Flowers |display-authors=etal |year=2019 |isbn=978-1-947-17262-3 |page=14 |access-date=February 17, 2023 |archive-date=February 17, 2023 |archive-url=https://web.archive.org/web/20230217173041/https://openstax.org/books/chemistry-2e/pages/1-2-phases-and-classification-of-matter |url-status=live }}</ref> However, advances in experimental techniques have revealed other previously theoretical phases, such as [[Bose–Einstein condensate]]s and [[fermionic condensate]]s.<ref>{{Cite web |title=The Nobel Prize in Physics 2001 |url=https://www.nobelprize.org/prizes/physics/2001/popular-information/ |access-date=February 17, 2023 |website=NobelPrize.org |language=en-US |archive-date=February 17, 2023 |archive-url=https://web.archive.org/web/20230217172801/https://www.nobelprize.org/prizes/physics/2001/popular-information/ |url-status=live }}</ref><ref>{{Cite book |last1=Cohen-Tannoudji |first1=Claude |url=https://books.google.com/books?id=HT_ICgAAQBAJ |title=Advances In Atomic Physics: An Overview |last2=Guery-Odelin |first2=David |date=2011 |publisher=World Scientific |isbn=978-981-4390-58-3 |pages=684 |language=en |author-link=Claude Cohen-Tannoudji |access-date=February 17, 2023 |archive-date=June 4, 2023 |archive-url=https://web.archive.org/web/20230604212103/https://books.google.com/books?id=HT_ICgAAQBAJ |url-status=live }}</ref>

Ordinary matter is composed of two types of [[elementary particle]]s: [[quark]]s and [[lepton]]s.<ref name="Hooft">{{cite book |author='t Hooft |first=G. |url=https://archive.org/details/insearchofultima0000hoof |title=In search of the ultimate building blocks |date=1997 |publisher=[[Cambridge University Press]] |isbn=978-0-521-57883-7 |page=[https://archive.org/details/insearchofultima0000hoof/page/6 6] |language=en |url-access=registration}}</ref> For example, the proton is formed of two [[up quarks]] and one [[down quark]]; the neutron is formed of two down quarks and one up quark; and the electron is a kind of lepton. An atom consists of an [[atomic nucleus]], made up of protons and neutrons (both of which are [[baryons]]), and electrons that orbit the nucleus.<ref name="OpenStax-college-physics">{{cite book |url=https://openstax.org/books/college-physics-2e/pages/33-4-particles-patterns-and-conservation-laws |title=College Physics 2e |publisher=OpenStax |first1=Paul Peter |last1=Urone |display-authors=etal |isbn=978-1-951-69360-2 |year=2022 |access-date=February 13, 2023 |archive-date=February 13, 2023 |archive-url=https://web.archive.org/web/20230213180410/https://openstax.org/books/college-physics-2e/pages/33-4-particles-patterns-and-conservation-laws |url-status=live }}</ref>{{rp|1476}} Because most of the mass of an atom is concentrated in its nucleus, which is made up of baryons, astronomers often use the term ''[[Baryon#Baryonic matter|baryonic matter]]'' to describe ordinary matter, although a small fraction of this "baryonic matter" is electrons.

Soon after the [[Big Bang]], primordial protons and neutrons formed from the [[quark–gluon plasma]] of the early universe as it cooled below two trillion degrees. A few minutes later, in a process known as [[Big Bang nucleosynthesis]], nuclei formed from the primordial protons and neutrons. This nucleosynthesis formed lighter elements, those with small atomic numbers up to [[lithium]] and [[beryllium]], but the abundance of heavier elements dropped off sharply with increasing atomic number. Some [[boron]] may have been formed at this time, but the next heavier element, [[carbon]], was not formed in significant amounts. Big Bang nucleosynthesis shut down after about 20 minutes due to the rapid drop in temperature and density of the expanding universe. Subsequent formation of [[metallicity|heavier elements]] resulted from [[stellar nucleosynthesis]] and [[supernova nucleosynthesis]].<ref name=Clayton1983>{{cite book|last1=Clayton|first1=Donald D.|title=Principles of Stellar Evolution and Nucleosynthesis|url=https://archive.org/details/principlesofstel0000clay|url-access=registration|date=1983|publisher=The University of Chicago Press|isbn=978-0-226-10953-4|pages=[https://archive.org/details/principlesofstel0000clay/page/362 362–435]}}</ref>