There are several proposed types of exotic 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 be exotic hadrons and within the Standard Model.
• States of matter that are not commonly encountered, such as Bose–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 and time crystal but whose properties are entirely within the realm of mainstream physics.
• Forms of matter that are poorly understood, such as dark matter and mirror matter.
• Ordinary matter that when placed under high 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 applied force. Despite being inconsistent with the expected behavior of "normal" matter, negative mass is mathematically consistent and introduces no violation of conservation of momentum or energy. It is used in certain speculative theories, such as on the construction of artificial wormholes and the Alcubierre drive. The closest known real representative of such exotic matter is the region of pseudo-negative-pressure density produced by the Casimir effect.

A hypothetical particle with complex rest mass would always travel faster than the speed of light. Such particles are called tachyons. 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 is observable and thus must be real. Therefore, the quantity under the square root must be negative, which can only happen if v is greater than c. As noted by Gregory Benford et al., special relativity implies that tachyons, if they existed, could be used to communicate backwards in time^[2] (see tachyonic antitelephone). Because time 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.

In quantum field theory, complex mass would induce tachyon condensation.

At high pressure, materials such as sodium chloride (NaCl) in the presence of an excess of either chlorine or sodium were transformed into compounds "forbidden" by classical chemistry, such as Na
₃Cl and NaCl
₃. Quantum mechanical calculations predict the possibility of other compounds, such as NaCl
₇, Na
₃Cl
₂ and Na
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 inside planetary 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 – Concept in cosmology
• 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 to Exotic matter.

• Exotic Matter and Negative Energy on YouTube
• The Riddle of AntiMatter on YouTube

v t e Particles in physics
Elementary	Fermions Quarks Up (quark antiquark) Down (quark antiquark) Charm (quark antiquark) Strange (quark antiquark) Top (quark antiquark) Bottom (quark antiquark) Leptons Electron Positron Muon Antimuon Tau Antitau Neutrino Electron neutrino Electron antineutrino Muon neutrino Muon antineutrino Tau neutrino Tau antineutrino Bosons Gauge Photon Gluon W and Z bosons Scalar Higgs boson Ghost fields Faddeev–Popov ghosts Hypothetical Superpartners Gauginos Gluino Gravitino Photino Others Axino Chargino Higgsino Neutralino Sfermion (Stop squark) Others Axion Curvaton Dilaton Dual graviton Graviphoton Graviton Inflaton Leptoquark Magnetic monopole Majoron Majorana fermion Dark photon Preon Sterile neutrino Tachyon W′ and Z′ bosons X and Y bosons
Composite	Hadrons Baryons Nucleon Proton Antiproton Neutron Antineutron Delta baryon Lambda baryon Sigma baryon Xi baryon Omega baryon Mesons Pion Rho meson Eta and eta prime mesons Bottom eta meson Phi meson J/psi meson Omega meson Upsilon meson Kaon B meson D meson Quarkonium Exotic hadrons Tetraquark (Double-charm tetraquark) Pentaquark Others Atomic nuclei Atoms Exotic atoms Positronium Muonium Tauonium Onia Pionium Protonium Superatoms Molecules Hypothetical Baryons Hexaquark Heptaquark Skyrmion Mesons Glueball Theta meson T meson Others Mesonic molecule Pomeron Diquark R-hadron
Quasiparticles	Anyon Davydov soliton Dropleton Exciton Fracton Hole Magnon Phonon Plasmaron Plasmon Polariton Polaron Roton Trion
Lists	Baryons Mesons Particles Quasiparticles Timeline of particle discoveries
Related	History of subatomic physics timeline Standard Model mathematical formulation Subatomic particles Particles Antiparticles Nuclear physics Eightfold way Quark model Exotic matter Massless particle Relativistic particle Virtual particle Wave–particle duality Particle chauvinism
Physics portal