Theories of everything attempt to unify this weakest of the 4 fundamental forces with electromagnetism, the weak, and the strong forces.
One theory of this force posits that quantum excitations in its underlying field form lines of force, which close up to form loops. Sources of light in areas where this field is strong will appear redshifted to an observer in an area where this field is weak. Any accelerating body can generate this type of wave, whose detection is the goal of LIGO. General relativity posits that this force is the result of the curvature of spacetime. Theories of everything attempt to unify this weakest of the 4 fundamental forces with electromagnetism, the weak, and the strong forces. Its universal constant has units of Newtons meters squared per kilogram squared. For 10 points, name this fundamental attractive force between objects with mass. 
One theory of this force posits that quantum excitations in its underlying field form lines of force, which close up to form loops.
gravity [accept word forms like gravitational force/field/waves etc]