Strange matter is matter consisting of atom s whose nuclei contain pairs of particles called strange quark s. Quarks are believed to be elementary particles, in the sense that they cannot be broken down into smaller components. There are several types of quarks, with exotic names such as up quark, down quark, and strange quark. These names were invented to differentiate among quarks with various characteristics. An atom of normal matter has a nucleus consisting of one or more proton s, and, with the exception of ordinary hydrogen, one or more neutron s. Protons and neutrons are made up of up quarks and down quarks in certain combinations. Strange matter, however, has nuclei that also contain strange quarks. This makes strange matter different from matter in the everyday sense. One of the notable characteristics of strange matter is its extreme density. A fragment of strange matter the size of a biological cell would have a mass of hundreds of kilograms. Scientists have produced atoms of strange matter in the laboratory, using a machine called a particle accelerator or atom smasher. Some astronomers believe that strange matter exists in interstellar and intergalactic space, particularly in dense objects such as stars that have spent all their atomic fuel and collapsed under the influence of their own gravitation. Such stars are optically invisible because they have stopped shining. If the universe contains many objects made up of strange matter, then there is far more mass in the cosmos than calculations based on visual observations suggest. If the existence of strange matter in large quantities can be proven, the fact could have a profound effect on theories concerning whether or not the universe is dense enough to eventually collapse under its own gravitation