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MOND: modified gravity instead of dark matter
1983· Theory
SpeculativeA serious idea with no experimental proof yet.
Mordehai Milgrom's 1983 proposal that gravity is stronger than Newton says at extremely weak pulls, explaining spinning galaxies without dark matter.
See it on the Science map →How it works
Below a tiny threshold, about a hundred-billionth of Earth's surface gravity, MOND says gravity fades with distance, not distance squared. That keeps outer stars orbiting fast, so galaxy rotation follows from visible mass alone. It predicted, in advance, a tight rule linking a galaxy's visible mass to its rotation speed raised to the fourth power, which holds remarkably well. But it struggles with galaxy clusters and the afterglow's pattern, and lacks a widely accepted version compatible with relativity.
Why it matters
It is the main rival to dark matter. Its success with galaxies, from one formula and one constant, is a real puzzle the dark-matter picture must explain.
How we know
Rotation curves of hundreds of galaxies fit with one constant. But clusters still need unseen mass, and the relativistic version TeVeS was ruled out in 2017, when gravitational waves and light from colliding neutron stars arrived together.
Still a mystery
Can any relativistic MOND fit the afterglow? A 2021 theory by Constantinos Skordis and Tom Złośnik claims to; whether it survives other tests is open.
Connections (3)
▸The Bullet Cluster: dark matter caught in the act is a hard test for MOND: modified gravity instead of dark matterWell supported
The relationship
In MOND, gravity comes only from visible matter, just strengthened at weak pulls. In the Bullet Cluster most visible matter is hot gas, so MOND expects lensing to peak on the gas. It peaks on the galaxies instead. MOND can survive only by adding unseen matter, such as heavy neutrinos, which gives up its main appeal of needing no invisible mass.
How the link was found
Clowe's team presented it as a test in 2006; the same year, Garry Angus, Benoit Famaey and HongSheng Zhao ('Can MOND take a bullet?') showed MOND fits only with extra unseen mass.
Is it proven?
The lensing offset is measured at 8σ. MOND already needed extra mass in ordinary clusters, so the Bullet sharpened an existing problem rather than creating a new one.
Open questions and mysteries
Could a sterile neutrino, a hypothetical heavier cousin of the known neutrinos, rescue MOND in clusters? Some newer MOND-like theories with extra fields claim to mimic the offset; tests continue.
▸Dark matter is replaced by modified gravity in MOND: modified gravity instead of dark matterSpeculative
The relationship
Galaxy rotation reveals a gap between visible mass and the gravity required. Dark matter fills the gap with unseen mass; MOND instead changes the force law, so the gap is not missing mass but missing gravity. MOND's strength is galaxies, where it predicts rotation from visible matter alone. Dark matter's strength is everything larger: clusters, the afterglow and the cosmic web.
How the link was found
Milgrom (1983) framed it as an alternative. Stacy McGaugh and Erwin de Blok showed in 1998 that MOND had correctly predicted rotation in dim, diffuse galaxies before it was measured.
Is it proven?
Neither is proven by direct detection. Most physicists favor dark matter because one ingredient explains many independent observations; MOND cannot fit the afterglow ripples without adding unseen matter.
Open questions and mysteries
Why does visible mass alone predict galaxy rotation so tightly, as McGaugh, Federico Lelli and James Schombert showed across 153 galaxies in 2016? Dark-matter models must reproduce that regularity.
▸MOND: modified gravity instead of dark matter changes, at very weak pull, Newton's law of gravitySpeculative
The relationship
In Newton's law, pull weakens with distance squared. MOND keeps that for strong pulls but, below about a hundred-billionth of Earth's surface gravity, makes the pull weaken only with distance. Galaxies' outer stars feel such weak pulls, so they get extra grip. The Sun's pull on Earth is tens of millions of times above the threshold, so Solar System tests see pure Newton.
How the link was found
Mordehai Milgrom, Weizmann Institute, Israel, in three 1983 papers. Jacob Bekenstein and Milgrom built a fuller version, derived from an energy principle, in 1984.
Is it proven?
Not proven. It matches galaxy rotation with one new constant. Widely separated star pairs, where pulls are weak, test it directly; Gaia-based studies in 2023–24 reached opposite conclusions, most favoring Newton.
Open questions and mysteries
Why would gravity change at that particular acceleration, about a sixth of the speed of light times the universe's expansion rate? Possibly a clue, possibly coincidence.