Exotic matter is a hypothetical concept of particle physics. It covers any material which violates one or more classical conitions or is not made of known baryonic particles. Such materials would possess qualities like negative mass or being repelled rather than attracted by gravity. It is used in certain speculative theories, such as on the construction of wormholes. The closest known real representative of exotic matter is a region of pseudo-negative pressure density produced by the Casimir effect.