I have before theoretized about the idea if matter with a negative mass could theoretically exist, and what it's properties would be, acc. to the standard theories (GR). In fact you could argue for two kinds of models for this hypothetical negative mass matter, namely: model-1: anti-symetric gravitational mass of negative matter = negative gravitational mass of positive (normal) matter. inertial mass of negative matter = intertial mass of positive (normal) matter. model-2: symmetric gravitational mass of negative matter = negative gravitational mass of positive (normal) matter. inertial mass of negative matter = negative intertial mass of positive (normal) matter. In a paper a Chinese physicus named Hyoyoung Choi discusses the possibilty that such kind of negative mass matter could exist (based on the symmetric model-2), and he argues that it could explain the dark matter component in the universe and also be responsible for the dark energy component of the universe. Just a weird idea that can be shown to be incorrect, or is he up to something imporant, and perhaps a major breaktrough in modern cosmology? See his paper here: Hypothesis of Dark Matter and Dark Energy with Negative Mass A discussion (but his english is unfortunately very poor) can be found here: Video of the motion of negative mass, dark matter, dark energy Summary of the idea: Negative mass matter, although a pure hypothetical proposition, is not a priori disallowed by the theory of General Relativity. If we use the symmetric model, in which the sign of the gravitational mass is equal to the sign of the inertial mass, we have the following properties of negative and positive masses: 1. Positive mass 1 and positive mass 2 Mutual attraction 2. Positive mass 1 and negative mass 2 Repulsion from mass 1 in the opposite direction of mass 2. Attraction from mass 2 in the direction of mass 1. 3. Negative mass 1 and negative mass 2 Mutual repulsion Examples: - If you have a ping pong sized negative mass it will fall to the earth just like a positive mass ping pong ball. - If you have two masses with equal quantity but different sign, the negative mass 'chases' the positive mass, and both accelerate in the same direction (Wow! great stuff for building space-ships, they will accelerate indefinately without any propulsion!!) - if you have a large distribution of only negative mass matter, the stuff will mutually repell and become evenly distributed in space. The hypothesis that this kind of 'dark matter' exists in the form of mass with a negative sign, and was formed (in equal amounts?) together with positive sign matter, would lead in our universe to: a. Firstly, a uniform distribution of the negative mass matter in the universe b. As positive matter clumps together (by both mutual attraction of the positive mass, and mutual repell on the negative mass - this would mean large structures could form earlier (?!)) the negative mass will cluster around galaxies, where it acts as the 'dark matter' component. c. The repelling force of the negative mass matter is the source of 'dark energy', which can be show to have caused the expansion and it's later accelerated expansion of spacetime.