Exotic matter is the name given to matter that has exotic properties, which means that it has some characteristics that are different from the usual physical properties of matter.

Most exotic matter is hypothetical, meaning that its existence is unobserved and unproved, but is speculated by theory. A type of hypothetical exotic matter is a particle with a negative mass. Because of Einstein’s theory of the equivalence between mass and energy, E=mc² , a particle of negative mass also has negative energy. A particle of this nature would behave in the opposite way as most particles. When people talk about exotic matter, they are usually referring to this type of matter with negative mass because this type of matter is thought to be necessary to keep wormholes open. A wormhole is a theoretical passage through space-time that could create shortcuts for travel across the universe. Unfortunately, wormholes have never been observed and are predicted to collapse very quickly. To stabilize them, it would require exotic matter that has yet to be discovered. Although popular in science fiction, using exotic matter to stabilize a wormhole for humans to travel or send information long distances is far beyond our current technological abilities.

Another hypothetical particle is called a tachyon. This particle has an imaginary number as its mass and can travel faster than the speed of light

Dark matter is sometimes considered a different type of hypothetical exotic matter. Most scientists accept that dark matter exists, but it is unobserved because it does not emit light or electromagnetic radiation that can be measured. In this case, the term "exotic matter" is being used in a broader sense to mean a particle whose properties are not well-understood. It could be the case that dark matter is made up of already known and normal-behaving particles, but we just don't know yet anything about it.

So far, I have only mentioned hypothetical exotic matter, but other types of exotic matter have been observed in reality. One example is what is called a Bose-Einstein condensate, which is a state of matter that occurs at extremely low temperatures near absolute zero. In a Bose-Einstein condensate, atoms clump together and behave as if the whole group were a single atom. This state of matter was predicted in 1924, but was finally created in the lab in 1995. This achievement won the 2001 Nobel Prize in Physics ( 2001 Nobel Prize in Physics ). In this case, the physics of Bose-Einstein condensation is expected from mainstream physics, but the state of matter is different from what we encounter under more ambient conditions, and so it sometimes gets the term "exotic." For example, the 2016 Nobel Prize in Physics went to theorists studying the mathematics of some exotic phases of matter ( 2016 Nobel Prize in Physics ).