There are several proposed types ofexotic matter:

• Hypothetical particles and states of matter that have not yet been encountered, but whose properties would be within the realm of mainstream physics if found to exist.
• Several particles whose existence has been experimentally confirmed that are conjectured to beexotic hadrons and within theStandard Model.
• States of matter that are not commonly encountered, such asBose–Einstein condensates,fermionic condensates,nuclear matter,quantum spin liquid,string-net liquid,supercritical fluid,color-glass condensate,quark–gluon plasma,Rydberg matter,Rydberg polaron,photonic matter,Wigner crystal,^[1]Superfluid andtime crystal but whose properties are entirely within the realm of mainstream physics.
• Forms of matter that are poorly understood, such asdark matter andmirror matter.
• Ordinary matter that when placed underhigh pressure, may result in dramatic changes in its physical or chemical properties.
• Degenerate matter
• Exotic atoms

Negative mass would possess some strange properties, such as accelerating in the direction opposite of appliedforce. Despite being inconsistent with the expected behavior of "normal" matter, negative mass is mathematically consistent and introduces no violation ofconservation of momentum orenergy. It is used in certain speculative theories, such as on the construction of artificialwormholes and theAlcubierre drive. The closest known real representative of such exotic matter is the region of pseudo-negative-pressure density produced by theCasimir effect.

A hypothetical particle with complex rest mass would always travel faster than thespeed of light. Such particles are calledtachyons. There is no confirmed existence of tachyons.

${\displaystyle E=\frac{m\cdot c^2}{\sqrt{1-\frac{{\left|\mathbf{v}\right|}^2}{c^2}}}}$

If the rest mass${\displaystyle m}$ is complex this implies that the denominator is complex because the total energy isobservable and thus must bereal. Therefore, the quantity under the square root must be negative, which can only happen ifv is greater thanc. As noted byGregory Benfordet al.,special relativity implies that tachyons, if they existed, could be used to communicate backwards in time^[2] (seetachyonic antitelephone). Becausetime travel is considered to be non-physical, tachyons are believed by physicists either not to exist, or else to be incapable of interacting with normal matter.

Inquantum field theory, complex mass would inducetachyon condensation.

At high pressure, materials such assodium chloride (NaCl) in the presence of an excess of either chlorine or sodium were transformed into compounds "forbidden" by classical chemistry, such asNa
₃Cl andNaCl
₃. Quantum mechanical calculations predict the possibility of other compounds, such asNaCl
₇,Na
₃Cl
₂ andNa
₂Cl. The materials are thermodynamically stable at high pressures. Such compounds may exist in natural environments that exist at high pressure, such as the deep ocean or insideplanetary cores. The materials have potentially useful properties. For instance,Na
₃Cl is a two-dimensional metal, made of layers of pure sodium and salt that can conduct electricity. The salt layers act as insulators while the sodium layers act as conductors.^[3][4]

• Antimatter – Material composed of antiparticles of the corresponding particles of ordinary matter
• Dark energy – Energy driving the accelerated expansion of the universe
• Dark matter – Hypothetical invisible cosmic material
• Gravitational interaction of antimatter – Theory of gravity on antimatter
• Mirror matter – Hypothetical counterpart to ordinary matter
• Negative energy – Concept in physics
• Negative mass – Concept in physical models
• QCD matter – Hypothetical phases of matter
• Strange matter – Degenerate matter made from strange quarks

Wikimedia Commons has media related toExotic matter.

• Exotic Matter and Negative Energy onYouTube
• The Riddle of AntiMatter onYouTube

• Exotic matter
• Hypothetical particles
• Gravity
• Wormhole theory
• Warp drive theory

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