Just guess

In theoretical physics, negative mass is matter whose mass is of opposite sign to the mass of normal matter, $\text{e.g (-2 k.g)}$ Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass. It is used in certain speculative theories, such as on the construction of wormholes. Originally the closest known real representative of such exotic matter is a region of negative pressure density produced by the Casimir effect. In 2017, researchers at Washington State University realized negative effective inertial mass experimentally by cooling rubidium atoms with lasers, although this is not negative mass in the fundamental sense. General relativity describes gravity and the laws of motion for both positive and negative energy particles, hence negative mass, but does not include the other fundamental forces. On the other hand, the Standard Model describes elementary particles and the other fundamental forces, but it does not include gravity. A unified theory that explicitly includes gravity along with the other fundamental forces may be needed for a better understanding of the concept of negative mass.