Space is filled with gravitons moving in every direction. These gravitons push masses together and cause "attraction". When gravitons approach a spiral galaxy parallel to its plane, they get pushed together and concentrated by the magnetic field lines of the galaxy. As they double their distance from the edge of the galaxy, the gravitons double their density and double the force they exert on a star. This allows stars to have higher velocities than we would expect them to have at a given distance from the centre of the galaxy and is in keeping with how gravity varies according to modified newtonian dynamics.