负物质拥有的质量是负数，被称为负质量（negative mass），由于根据相对论质量和能量可互相转换，负质量在某种意义上又可以等同负能量。注意这里的「负物质」与「反物质」（antimatter）是完全不同的概念，负物质拥有负质量/负能量，而反物质具有正质量/正能量。反物质与普通物质一样会被重力场吸引，但另一方面负物质不会受重力场吸引，反而会受其排斥。

物理学家米给尔·阿库别瑞就曾提出一个阿库别瑞引擎（或称曲速引擎），需要由负物质或负能量推动。

负物质仍在搜索阶段，但负能量的确是存在的。卡西米尔效应的实验中，当两块金属片距离遥远时，能量为零。当它们彼此靠近，我们能从中吸取能量，其时金属板的能量即为负值。

In theoretical physics, negative mass is a hypothetical concept of matter whose mass is of opposite sign to the mass of normal matter, e.g. −2 kg. 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. The closest known real representative of such exotic matter is a region of pseudo-negative pressure density produced by the Casimir effect. Although general relativity well describes gravity and the laws of motion for both positive and negative energy particles, hence negative mass, it does not include the other fundamental forces. On the other hand, although the Standard Model well describes elementary particles and the other fundamental forces, it does not include gravity, even though gravity is intimately involved in the origin of mass and inertia. A model that explicitly includes gravity along with the other fundamental forces may be needed for a better understanding of the concept of negative mass.