Unsourced or poorly sourced material may be challenged and removed. Please add such references to provide context and establish the relevance of any primary research articles cited. This section needs additional citations to secondary or tertiary sources such as review articles, monographs, or textbooks. Of the possible alternatives, one of the most notable is modified Newtonian dynamics (MOND), which involves modifying the laws of gravity. Though dark matter is by far the most accepted explanation of the rotation problem, other proposals have been offered with varying degrees of success. A solution to this conundrum is to hypothesize the existence of dark matter and to assume its distribution from the galaxy's center out to its halo. When mass profiles of galaxies are calculated from the distribution of stars in spirals and mass-to-light ratios in the stellar disks, they do not match with the masses derived from the observed rotation curves and the law of gravity.
The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material. The mass estimations for galaxies based on the light they emit are far too low to explain the velocity observations. This reflects the mass distributions within those systems. In contrast, the orbital velocities of planets in planetary systems and moons orbiting planets decline with distance according to Kepler’s third law. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. The rotational/orbital speeds of galaxies/stars do not follow the rules found in other orbital systems such as stars/planets and planets/moons that have most of their mass at the centre. Theories involving dark matter are the main postulated solutions to account for the variance.
A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. Right: Galaxy with a flat rotation curve that would be expected with dark matter. Left: A simulated galaxy without dark matter.
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