Modified Newtonian Dynamics (MOND) and its relativistic extension, Tensor-Vector-Scalar gravity (TeVeS), propose modifications to gravitational laws at low accelerations as alternatives to dark matter.
A modification of gravity at low accelerations
In 1983, Israeli physicist Mordehai Milgrom proposed Modified Newtonian Dynamics (MOND) to explain galaxy rotation curves without invoking dark matter. MOND suggests that Newton's laws of gravity need modification at extremely low accelerations - typically below , which is about one ten-billionth of Earth's surface gravity.
At these low accelerations, which are common in the outer regions of galaxies, MOND proposes that the gravitational force becomes proportional to the square of the distance rather than following the inverse-square law. This simple modification explains flat rotation curves naturally, without requiring additional unseen matter.
Rather than viewing dark matter as a substance, MOND reimagines it as a modification to our understanding of how gravity behaves at certain scales. This approach follows the historical pattern where apparent anomalies (like Mercury's orbit) led to deeper understanding of physical laws (General Relativity) rather than new substances.