List of unsolved problems in physics

A compilation of major questions in physics that lack agreed answers, spanning cosmology, quantum theory, particle physics, and condensed matter.

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Dark matter: Matter inferred from gravitational effects that does not emit, absorb, or reflect enough light to be seen directly. Its identity remains unknown despite strong evidence for its gravitational influence.
Standard Model: The quantum field theory describing known elementary particles and three of the four fundamental interactions. Its successes make its unexplained parameters and omissions especially striking.
Event Horizon Telescope: A global radio telescope array that produces horizon-scale images of black holes. Its observations test strong-gravity predictions but do not settle quantum black-hole physics.
ΛCDM model: The standard cosmological model containing a cosmological constant and cold dark matter. Its predictive successes coexist with unresolved questions about dark matter and dark energy.
Cosmic inflation: A proposed period of extremely rapid expansion in the universe’s earliest moments. Its mechanism and distinctive observational signatures remain under investigation.
Dark energy: A name for whatever drives the observed accelerated expansion of the universe. Its physical nature is not established, though its effects shape cosmic expansion.
General relativity: Einstein’s theory of gravity, in which matter and energy curve spacetime. Its incompatibility with quantum theory motivates the search for quantum gravity.
LIGO: A laser interferometer observatory that detects gravitational waves. Gravitational-wave measurements offer new tests of gravity and compact-object physics.
MOND: A modified-gravity framework proposed to explain galactic dynamics without invoking dark matter in some settings. It offers a rival account of selected gravitational evidence attributed to dark matter.
Black hole information paradox: The conflict between black-hole evaporation and quantum theory’s conservation of information. Resolving it could reveal how quantum mechanics and gravity fit together.