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Matter is commonly defined as being anything that has mass and that takes up space. However this definition is ambiguous, and leads to some problems, leading some physicists to define matter in terms of certain types of elementary particles. "Normal matter" constitutes about 5% of the mass of the observable universe, the remaining mass being composed of exotic and poorly understood forms of mass, currently known as dark matter and dark energy. There are four phases of macroscopic matter (solid, liquid, gas, and plasma) although some exotic phases (such as Bose-Einstein condensates and quark-gluon plasma) exist as well. The common definition of matter is anything which both occupies space and has mass. For example, a car would be said to be made of matter, as it occupies space, and has mass. In chemistry, this is often taken to mean what atoms and molecules are made of, meaning anything made of protons, neutrons, and electrons. For example, phosphorus sesquisulfide is a molecule made of four atoms of phosphorus, and three of sulfur (see image on right), and is thus considered to be matter. However in physics, there is no broad consensus as to an exact definition of matter, partly because the notion of "taking up space" is ambiguous in quantum mechanics, and partly because mass doesn't lead to a "natural classification" of particles. Therefore physicists generally do not use the term matter when precision is needed, preferring instead to speak of the more clearly defined concepts of mass, energy, and particles. In discussions of matter and antimatter, normal matter is also sometimes referred to as koinomatter.[1]
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