MOND

MOND is the short name for Modified Newtonian Dynamics. It is one of the possible answers along with dark matter for an explanation of why we stars and galaxies are moving as they do (something more is needed as the laws and theories existing prior do not properly explain what is seen). It is a modification of Newtonian force law that was hypothesized in 1983 as an alternative to dark matter.

The modification can be interpreted as either a modification to gravity or a modification to inertia. This modification occurs at very small accelerations. Above a critical acceleration (a₀), everything is normal. Below a₀, the effective acceleration approaches a = (g_N a₀)^1/2, where g_N is the normal Newtonian acceleration. a₀ = 1.2 * 10^-10m/s^-2, or about one angstrom per second per second. This is one part in 10¹¹ of the gravity that is felt on Earth.

MOND was designed to fit the rotation curves of spiral galaxies that are currently explained with dark matter. It is often claimed that MOND is ad hoc and thus bad. However, this is also true of dark matter which was hypothesized to explain the rotational curves of spiral galaxies. One of the advantages of MOND over dark matter is that it does not cause any extra problems with a Newtonian disk and a dark matter halo.

MOND does not contradict General Relativity as they apply to different ends of the extremes of velocity. Relativity is what happens as v -> c, while MOND is what happens as a -> 0.

Empirically, MOND works. There is no empirical evidence that clearly contradicts it. Its predictions come true where those of dark matter based theories fail.

This is more of a comment,
There is every evidence that there really is no problem with having a Newtonian disk, with or without a dark matter halo and without MOND.

Dark matter haloes were introduced in the 1970's to explain the stability of Newtonian disks but if you do a full hydro code (this is a piece of software that simulates the dynamics of a large number of particles by solving the hydrodyanmical equations of motion for the gas whilst also including gravity and collisionless matter such as dark matter. The codes accuracy depends very much on the number of particles that are simulated, in the most modern codes circa 2000 only a few million particles can be simulated. The hope is that this is enough to begin to show the dynamics of real objects galactic sized objects. These objects usually contain many orders of magnitude of particles than can be simulated. An analogy would be attempting to model the Atlantic using a basin of water.) which follows collisons accuratly then you can have a stable disk without a halo. (it was later noted that halos explained the rotation curves of disk galaxies) (see disk galaxy)