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This article is about the concept in astronomy, physics and chemistry. For other uses, see Matter .

The term matter traditionally refers to the substance that all objects are made of. One common way to identify this "substance" is through its physical properties; a common definition of matter is anything that has mass and occupies a volume. However, this definition has to be revised in light of quantum mechanics, where the concept of "having mass", and "occupying space" are not as well-defined as in everyday life. A more general view is that bodies are made of several substances, and the properties of matter (among them, mass and volume) are determined not only by the substances themselves, but by how they interact. In other words, matter is made up of interacting "building blocks", the so-called particulate theory of matter.

The concept of matter has been refined many times in history, in light of the improvement in knowledge of just what the basic building blocks are, and in how they interact. For example, in the early 18th century, Isaac Newton viewed matter as "solid, massy, hard, impenetrable, movable particles", which were "even so very hard as never to wear or break in pieces" The "primary" properties of matter were amenable to mathematical description, unlike "secondary" qualities such as color or taste. In the 19th century, following the development of the periodic table, and of atomic theory, atoms were seen as being the fundamental constituents of matter; atoms formed molecules and compounds.

In the late 19th century with the discovery of the electron, and in the early 20th century, with the discovery of the atomic nucleus, and the birth of particle physics, matter was seen as made up of electrons, protons and neutrons interacting to form atoms. Today, we know that even protons and neutrons are not indivisible, they can be divided into quarks, while electrons are part of a particle family called leptons. Both quarks and leptons are elementary particles, and are currently seen as being more fundamental constituents of matter.

These quarks and leptons interact through four fundamental forces: gravity, electromagnetism, weak interactions, and strong interactions. The Standard Model of particle physics is currently the best explanation for all of physics, but despite decades of efforts, gravity cannot yet be accounted for at the quantum-level; it is only described by classical physics (see quantum gravity and graviton). Interactions between quarks and leptons are the result of an exchange of force-carrying particles (such as photons) between quarks and leptons. The force-carrying particles are not themselves building blocks. As one consequence, mass and energy cannot always be related to matter. For example, the carriers of the electric force (photons) possess energy (see Planck relation) and the carriers of the weak force (W bosons) are massive, but neither are considered matter either. However, while these particles are not considered matter, they do contribute to the total mass of atoms or subatomic particles.

Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental technique have realized other phases, previously only theoretical constructs, such as Bose–Einstein condensates and Fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark-gluon plasma.

In physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave-particle duality.

In the realm of cosmology, extensions of the term matter are invoked to include dark matter and dark energy, concepts introduced to explain some odd phenomena of the observable universe, such as the galactic rotation curve. These exotic forms of "matter" do not refer to matter as "building blocks", but rather to currently poorly-understood forms of mass and energy.


Common definition

The common definition of matter is anything that has both mass and volume (occupies space). For example, a car would be said to be made of matter, as it occupies space, and has mass.

The observation that matter occupies space goes back to antiquity. However, an explanation for why matter occupies space is recent, and is argued to be a result of the Pauli exclusion principle. Two particular examples where the exclusion principle clearly relates matter to the occupation of space are white dwarf stars and neutron stars, discussed further below.

Amount of substance

The international standards organization Bureau International des Poids et Mesures (BIPM) uses the terminology "amount of substance", rather than "matter". To quote the SI brochure:
"Amount of substance is defined to be proportional to the number of specified elementary entities in a sample, the proportionality constant being a universal constant which is the same for all samples.
The unit of amount of substance is called the mole, symbol mol, and the mole is defined by specifying the mass of carbon 12 that constitutes one mole of carbon 12 atoms.
By international agreement this was fixed at 0.012 kg, i.e.
12 g.
  • 1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is "mol".
  • 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."

Atoms and molecules definition

A definition of "matter" that is based upon its physical and chemical structure is: matter is made up of atoms and molecules. This definition is consistent with the BIPM definition of "amount of substance" above, but is more specific about the constituents of matter (and unconcerned about the unit mole). Further discussion appears below in the discussion section and in the description of the quarks and leptons definition. As an example of matter under this definition, genetic information is carried by a long molecule called DNA, which is copied and inherited across generations. It is matter under this definition because it is made of atoms, not by virtue of having mass or occupying space. This definition can be extended to include charged atoms and molecules, so as to include plasma (gases of ions) and electrolytes (ionic solutions), which are not obviously included in the atoms and molecules definition. Alternatively, one can adopt the protons, neutrons and electrons definition below.

Protons, neutrons and electrons definition

A definition of "matter" more fine-scale than the atoms and molecules definition is: matter is made up of what atoms and molecules are made of, meaning anything made of protons, neutrons, and electrons. This definition goes beyond atoms and molecules, however, to include substances made from these building blocks that are not simply atoms or molecules, for example white dwarf matter — typically, carbon and oxygen nuclei in a sea of degenerate electrons. At a microscopic level, the constituent "particles" of matter such as protons, neutrons and electrons obey the laws of quantum mechanics and exhibit wave-particle duality. At an even deeper level, protons and neutrons are made up of quarks and the force fields (gluons) that bind them together (see Quarks and leptons definition below).

Quarks and leptons definition

As may be seen from the above discussion, many early definitions of what can be called ordinary matter were based upon its structure or "building blocks". On the scale of elementary particles, a definition that follows this tradition can be stated as: ordinary matter is everything that is composed of elementary fermions, namely quarks and leptons. The connection between these formulations follows.

Leptons (the most famous being the electron), and quarks (of which baryons, such as protons and neutrons, are made) combine to form atoms, which in turn form molecules. Because atoms and molecules are said to be matter, it is natural to phrase the definition as: ordinary matter is anything that is made of the same things that atoms and molecules are made of. (However, notice that one also can make from these building blocks matter that is not atoms or molecules.) Then, because electrons are leptons, and protons and neutrons are made of quarks, this definition in turn leads to the definition of matter as being "quarks and leptons", which are the two types of elementary fermions. Carithers and Grannis state: Ordinary matter is composed entirely of first-generation particles, namely the u [up] and d [down] quarks, plus the electron and its neutrino. (By "first-generation" is meant the stable quarks and leptons. Higher "generations" decay into "first-generation" particles.)

This definition of ordinary matter is more subtle than it first appears. There are two groups of particles. All the particles that make up matter, such as electrons, protons and neutrinos, are fermions. All the force carriers are bosons. See the tabulation in the figure. The W and Z bosons that mediate the weak force are not made of quarks and leptons, and so are not ordinary matter, but do have mass. In other words, mass is not something that is exclusive to ordinary matter.

The quark-lepton definition of ordinary matter, however, identifies not only the elementary building blocks of matter, but also includes composites made from the constituents (atoms and molecules, for example). Such composites contain an interaction energy that holds the constituents together, and may constitute the bulk of the mass of the composite. As an example, to a great extent, the mass of an atom is simply the sum of the masses of its constituent protons, neutrons and electrons. However, digging deeper, the protons and neutrons are made up of quarks bound together by gluon fields (see QCD). Basically, much of the mass of hadrons is the interaction energy of bound quarks. Thus, most of what composes the "mass" of ordinary matter is interquark interaction energy. For example, "the gluonic forces binding three quarks (total mass 12.5 MeV) to make a nucleon contribute most of its mass of 938 MeV". In a similar vein, the quark gluon plasma is considered to be a state of matter, and obviously includes the gluons. The bottom line here is: in a complex such as an atom or a hadron, the matter in the complex is generally not the most significant source of the mass belonging to the complex.

Smaller building blocks?

“In the past, the search for building blocks of matter has led us to more and more 'elementary' entities – from the molecule to the atom, to the nucleus and electrons, to the nucleons, and eventually to the quarks. Have we completed this 'onion peeling' process ... ?” The Standard Model groups matter particles into three generations, where each generation consists of two quarks and two leptons. The first generation is the up and down quarks, the electron and the electron neutrino; the second includes the charm and strange quarks, the muon and the muon neutrino; the third generation consists of the top and bottom quarks and the tau and tau neutrino. “... the most natural explanation to the existence of higher generations of quarks and leptons is that they correspond to excited states of the first generation, and experience suggests that excited systems must be composite.”

Discussion and background

The common definition in terms of occupying space and having mass is in contrast with most physical and chemical definitions of matter, which rely instead upon its structure and upon attributes not necessarily related to volume and mass. James Clerk Maxwell discussed matter in his work Matter and Motion. He carefully separates "matter" from space and time, and defines it in terms of the object referred to in Newton's first law of motion. In the 19th century, the term "matter" was actively discussed by a host of scientists and philosophers, and a brief outline can be found in Levere. A textbook discussion from 1870 suggests matter is what is made up of atoms:
Three divisions of matter are recognized in science: masses, molecules and atoms.

A Mass of matter is any portion of matter appreciable by the senses.

A Molecule is the smallest particle of matter into which a body can be divided without losing its identity.

An Atom is a still smaller particle produced by division of a molecule.

Rather than simply having the attributes of mass and occupying space, matter was held to have chemical and electrical properties. The famous physicist J. J. Thomson wrote about the "constitution of matter" and was concerned with the possible connection between matter and electrical charge. There is an entire literature concerning the "structure of matter", ranging from the "electrical structure" in the early 20th century, to the more recent "quark structure of matter", introduced today with the remark: Understanding the quark structure of matter has been one of the most important advances in contemporary physics. In this connection, physicists speak of matter fields, and speak of particles as "quantum excitations of a mode of the matter field". And here is a quote from De Sabbata and Gasperini: "With the word "matter" we denote, in this context, the sources of the interactions, that is spinor fields (like quarks and leptons), which are believed to be the fundamental components of matter, or scalar field, like the Higgs particles, which are used to introduced mass in a gauge theory (and which, however, could be composed of more fundamental fermion fields)."

The term "matter" is used throughout physics in a bewildering variety of contexts: for example, one refers to "condensed matter physics", "elementary matter", "partonic" matter, "dark" matter, "anti"-matter, "strange" matter, and "nuclear" matter. In discussions of matter and antimatter, normal matter has been referred to by Alfvén as koinomatter. It is fair to say that in physics, there is no broad consensus as to an exact definition of matter, and the term "matter" usually is used in conjunction with some modifier.

Phases of ordinary matter

In bulk, matter can exist in several different forms, or states of aggregation, known as phases, depending on ambient pressure, temperature and volume. A phase is a form of matter that has a relatively uniform chemical composition and physical properties (such as density, specific heat, refractive index, and so forth). These phases include the three familiar ones (solids, liquids, and gases), as well as more exotic states of matter ( such as plasma, superfluids, supersolids, Bose-Einstein condensates, ...). A fluid may be a liquid, gas or plasma. There are also paramagnetic and ferromagnetic phases of magnetic materials. As conditions change, matter may change from one phase into another. These phenomena are called phase transitions, and are studied in the field of thermodynamics. In nanomaterials, the vastly increased ratio of surface area to volume results in matter that can exhibit properties entirely different from those of bulk material, and not well described by any bulk phase (see nanomaterials for more details).

Phases are sometimes called states of matter, but this term can lead to confusion with thermodynamic states. For example, two gases maintained at different pressures are in different thermodynamic states (different pressures), but in the same phase (both are gases).


Solids are characterized by a tendency to retain their structural integrity; if left on their own, they will not spread in the same way gas or liquids would. Many solids, like rocks and concrete, have very high hardness and rigidity and will tend to break or shatter when subject to various forms of stress, but others like steel and paper are more flexible and will bend. Solids are often composed of crystals, glasses, or long chain molecules (e.g. rubber and paper). Some solids are amorphous such as glass. A common example of a solid is the solid form of water, ice.


In a liquid, the constituents frequently are touching, but able to move around each other. So unlike a gas, it has cohesion and viscosity. Compared to a solid, the forces holding constituents together are weaker, and it is not rigid, but adapts a shape decided by its container. Liquids are hard to compress. A common example is water.


A gas is a state of aggregation without cohesion; a vapor. Thus a gas has no resistance to changing shape (beyond the inertia of its constituents, which have to be knocked aside). The distance between constituent particles is flexible, determined, for example, by the size of a container and the number of particles, not by internal forces. A common example is the vapor form of water, steam.


Plasma is a fourth state of matter consisting of an overall charge-neutral mix of electrons, ions and neutral atoms. The plasma exhibits behavior peculiar to long range Coulomb forces in which the particles move in electromagnetic fields generated by and self-consistent with their own motions. The sun and stars are plasmas, as is the Earth's ionosphere, and plasmas occur in neon signs. Plasmas of deuterium and tritium ions are used in fusion reactions. The term plasma was applied for the first time by Tonks and Langmuir in 1929, to the inner regions of a glowing ionized gas produced by electric discharge in a tube.

Bose–Einstein condensate

This state of matter was first discovered by Satyendra Nath Bose, who sent his work on statistics of photons to Albert Einstein for comment. Following publication of Bose's paper, Einstein extended his treatment to massive particles fixed in number, and predicted this fifth state of matter in 1925. Bose–Einstein condensates were first realized experimentally by several different scientific groups in 1995 for rubidium, sodium, and lithium, using a combination of laser and evaporative cooling. Bose–Einstein condensation for atomic hydrogen was achieved in 1998.

The Bose–Einstein condensate is a liquid-like superfluid that occurs at low temperatures in which all atoms occupy the same quantum state. In low-density systems, it occurs at or below 10−5 K.

Fermionic condensate

A fermonic condensate is a superfluid phase formed by fermionic particles at low temperatures. It is closely related to the Bose-Einstein condensate under similar conditions. Unlike the Bose-Einstein condensates, fermionic condensates are formed using fermions instead of bosons. The earliest recognized fermionic condensate described the state of electrons in a superconductor; the physics of other examples including recent work with fermionic atoms is analogous. The first atomic fermionic condensate was created by Deborah S. Jin in 2003. These atomic fermionic condensates are studied at temperatures in the vicinity of 50-350 nK.

A hypothetical fermionic condensate that appears in theories of massless fermions with chiral symmetry breaking is the chiral condensate or the quark condensate.

Core of a neutron star

Because of its extreme density, the core of a neutron star falls under no other state of matter. While a white dwarf is about as massive as the sun (up to 1.4 solar masses, the Chandrasekhar limit), the Pauli exclusion principle prevents its collapse to smaller radius, and it becomes an example of degenerate matter. In contrast, neutron stars are between 1.5 and 3 solar masses, and achieve such density that the protons and electrons are crushed to become neutrons. Neutrons are fermions, so further collapse is prevented by the exclusion principle, forming so-called neutron degenerate matter.

Phases of nuclear matter; Compare with Siemens & Jensen.

Quark-gluon plasma

Gluons are elementary particles that cause quarks to interact, and are indirectly responsible for the binding of protons and neutrons together in atomic nuclei. The quark-gluon plasma is a hypothetical phase of matter, a phase of matter as yet not observed, supposed to exist in the early universe and to have evolved into a hadronic-gas phase. At extremely high energy the strong force is anticipated to become so weak that the atomic nuclei break down into a bunch of loose quarks, which distinguishes the quark-gluon phase from normal plasma. In collisions of relativistic heavy ions, a phase transition occurs from the nuclear, hadronic phase to a matter phase consisting of quarks and gluons. So far, experimental results have shown that instead of a weakly interacting plasma, an almost ideal liquid is produced. An animation is found at Gold ion collision @ RHIC.

Transparent Aluminum

In 2009, scientists from Oxford Universitymarker led an international team in using the FLASH laser synchrotron in Hamburg, Germanymarker to create a new state of matter, transparent aluminum. Using a short pulse from the FLASH laser, they removed a core electron from each aluminium atom, but did not destroy or disrupt the metal’s crystalline structure. What resulted was an aluminum that was almost invisible to ultraviolet radiation. Scientists involved in the discovery suggest that this will aid in further research concerning planetary science and nuclear fusion. The effect on the aluminum lasted for 40 femtoseconds.

A concept of transparent aluminum was seen in Star Trek IV.

Structure of ordinary matter

In particle physics, fermions are particles which obey Fermi–Dirac statistics. Fermions can be elementary, like the electron, or composite, like the proton and the neutron. In the Standard Model there are two types of elementary fermions: quarks and leptons, which are discussed next.


Quarks are a particles of spin- , implying that they are fermions. They carry an electric charge of −  e (down-type quarks) or +  e (up-type quarks). For comparison, an electron has a charge of −1 e. They also carry colour charge, which is the equivalent of the electric charge for the strong interaction. Quarks also undergo radioactive decay, meaning that they are subject to the weak interaction. Quarks are massive particles, and therefore are also subject to gravity.

Quark properties
Name Symbol Spin Electric charge


Mass comparable to Antiparticle Antiparticle

Up-type quarks
Up + 1.5 to 3.3 ~ 5 electrons Antiup
Charm + 1160 to 1340 ~ 1 proton Anticharm
Top + 169,100 to 173,300 ~ 180 protons or

~ 1 tungsten atom
Down-type quarks
Down 3.5 to 6.0 ~ 10 electrons Antidown
Strange 70 to 130 ~ 200 electrons Antistrange
Bottom 4130 to 4370 ~ 5 protons Antibottom

Quark structure of a proton: 2 up quarks and 1 down quark.

Baryonic matter

Baryons are strongly interacting fermions, and so are subject to Fermi-Dirac statistics. Amongst the baryons are the protons and neutrons, which occur in atomic nuclei, but many other unstable baryons exist as well. The term baryon is usually used to refer to triquarks — particles made of three quarks. "Exotic" baryons made of four quarks and one antiquark are known as the pentaquarks, but their existence is not generally accepted.

Baryonic matter is the part of the universe that is made of baryons (including all atoms). This part of the universe does not include dark energy, dark matter, black holes or various forms of degenerate matter, such as compose white dwarf stars and neutron stars. Microwave light seen by Wilkinson Microwave Anisotropy Probe (WMAP), suggests that only about 4.6% of that part of the universe within range of the best telescopes (that is, matter that may be visible because light could reach us from it), is made of baryionic matter. About 23% is dark matter, and about 72% is dark energy.

Degenerate matter

In physics, degenerate matter refers to the ground state of a gas of fermions at a temperature near absolute zero. The Pauli exclusion principle requires that only two fermions can occupy a quantum state, one spin-up and the other spin-down. Hence, at zero temperature, the fermions fill up sufficient levels to accommodate all the available fermions, and for the case of many fermions the maximum kinetic energy called the Fermi energy and the pressure of the gas becomes very large and dependent upon the number of fermions rather than the temperature, unlike normal states of matter.

Degenerate matter is thought to occur during the evolution of heavy stars. The demonstration by Subrahmanyan Chandrasekhar that white dwarf stars have a maximum allowed mass because of the exclusion principle caused a revolution in the theory of star evolution.

Degenerate matter includes the part of the universe that is made up of neutron stars and white dwarfs.

Strange matter

Strange matter is a particular form of quark matter, usually thought of as a 'liquid' of up, down, and strange quarks. It is to be contrasted with nuclear matter, which is a liquid of neutrons and protons (which themselves are built out of up and down quarks), and with non-strange quark matter, which is a quark liquid containing only up and down quarks. At high enough density, strange matter is expected to be color superconducting. Strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers (quark stars).

Two meanings of the term "strange matter"
In particle physics and astrophysics, the term is used in two ways, one broader and the other more specific.
  1. The broader meaning is just quark matter that contains three flavors of quarks: up, down, and strange. In this definition, there is a critical pressure and an associated critical density, and when nuclear matter (made of protons and neutrons) is compressed beyond this density, the protons and neutrons dissociate into quarks, yielding quark matter (probably strange matter).
  2. The narrower meaning is quark matter that is more stable than nuclear matter. The idea that this could happen is the "strange matter hypothesis" of Bodmer and Witten . In this definition, the critical pressure is zero: the true ground state of matter is always quark matter. The nuclei that we see in the matter around us, which are droplets of nuclear matter, are actually metastable, and given enough time (or the right external stimulus) would decay into droplets of strange matter, i.e. strangelets.


Leptons are a particles of spin- , meaning that they are fermions. They carry an electric charge of −1 e (electron-like leptons) or 0 e (neutrinos). Unlike quarks, leptons do not carry colour charge, meaning that they do not experience the strong interaction. Leptons also undergo radioactive decay, meaning that they are subject to the weak interaction. Leptons are massive particles, therefore are subject to gravity.

Lepton properties
Name Symbol Spin Electric charge


Mass comparable to Antiparticle Antiparticle

Charged leptons
Electron −1 0.5110 1 electron Antielectron

Muon −1 105.7 ~ 200 electrons Antimuon
Tauon −1 1,777 ~ 2 protons Antitauon
Electron neutrino 0 0.000460 Less than a thousandth of an electron Electron antineutrino
Muon neutrino 0 0.19 Less than half of an electron Muon antineutrino
Tauon neutrino

(or tau neutrino)
0 18.2 Less than ~ 40 electrons Tauon antineutrino

(or tau antineutrino)


In particle physics and quantum chemistry, antimatter is matter that is composed of the antiparticles of those that constitute ordinary matter. If a particle and its antiparticle come into contact with each other, the two annihilate; that is, they may both be converted into other particles with equal energy in accordance with Einstein's equation . These new particles may be high-energy photons (gamma rays) or other particle–antiparticle pairs. The resulting particles are endowed with an amount of kinetic energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original particle-antiparticle pair, which is often quite large.

Antimatter is not found naturally on Earth, except very briefly and in vanishingly small quantities (as the result of radioactive decay or cosmic rays). This is because antimatter which came to exist on Earth outside the confines of a suitable physics laboratory would almost instantly meet the ordinary matter that Earth is made of, and be annihilated. Antiparticles and some stable antimatter (such as antihydrogen) can be made in tiny amounts, but not in enough quantity to do more than test a few of its theoretical properties.

There is considerable speculation both in science and science fiction as to why the observable universe is apparently almost entirely matter, and whether other places are almost entirely antimatter instead. In the early universe, it is thought that matter and antimatter were equally represented, and the disappearance of antimatter requires an asymmetry in physical laws called the charge parity (or CP symmetry) violation. CP symmetry violation can be obtained from the Standard Model, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. Possible processes by which it came about are explored in more detail under baryogenesis.

Other types of matter

Ordinary matter, in the quarks and leptons definition, constitutes about 4% of the energy of the observable universe. The remaining energy is theorized to be due to exotic forms, of which 23% is dark matter and 73% is dark energy.

Dark matter

In astrophysics and cosmology, dark matter is matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. Observational evidence of the early universe and the big bang theory require that this matter have energy and mass, but is not composed of either elementary fermions (as above) OR gauge bosons. The commonly accepted view is that most of the dark-matter is non-baryonic in nature. As such, it is composed of particles as yet unobserved in the laboratory. Perhaps they are supersymmetric particles, which are not Standard Model particles, but relics formed at very high energies in the early phase of the universe and still floating about.

Dark energy

In cosmology, dark energy is the name given to the antigravitating influence that is accelerating the rate of expansion of the universe. It is known not to be composed of known particles like protons, neutrons or electrons, nor of the particles of dark matter, because these all gravitate.

Exotic matter

Exotic matter is a hypothetical concept of particle physics. It covers any material which violates one or more classical conditions or is not made of known baryonic particles. Such materials would possess qualities like negative mass or being repelled rather than attracted by gravity.


  1. The particulate theory of matter dates back to Leucippus (≈490 BC) and Democritus (≈470-380 BC). See
  2. Newton's 31st query, as quoted by
  3. The history of the concept of matter is a history of the fundamental length scales used to define matter. Different building blocks apply depending upon whether one defines matter on an atomic or elementary particle level. One may use a definition that matter is atoms, or that matter is hadrons, or that matter is leptons and quarks depending upon the scale at which one wishes to define matter.
  4. See for example, , and .
  5. See p.7 in
  6. The W boson mass is 80.43 GeV; see Figure 1 in
  7. RHIC Scientists Serve Up "Perfect" Liquid
  8. A. Bodmer "Collapsed Nuclei" Phys. Rev. D4, 1601 (1971)
  9. E. Witten, "Cosmic Separation Of Phases" Phys. Rev. D30, 272 (1984)

Further reading

External links

See also

Dark matter




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