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" 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 (liquid, gas, solid, and plasma) although some exotic phases exists (like Bose-Einstein condensates) as well.

Common definition

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 koinomatterFact: date=October 2008.

Mass definition

Since space is problematic, a possible definition of matter could be anything that has mass. This leads to some inelegance problems in particle physics, as particles tend to be regrouped into "families" based on properties other than mass. For example, photons (which have no mass) and W bosons (which have mass) are both gauge bosons.

Quarks and leptons definition

A possible definition of matter, which at least some physicists use, is that matter is everything that is composed of elementary fermions, namely quarks and leptons.Povh, Rith, Scholz, Zetche, Reigthinger Particles and Nuclei, 1999, ISBN 3540438238 Leptons (the most famous being the electron), and quarks (of which baryons, such as protons and neutrons, are made) combine to form atoms, which in turn forms molecules. Since atoms and molecules are said to be matter, it is natural to generalize what matter is as being anything that is made of the same things that atoms and molecules are made of. Since electrons are leptons, and protons and neutrons are made of quarks, this leads to the definition of matter as being "quarks and leptons", which are the two elementary types of fermions.

This definition of matter means that mass is not something that is exclusive to matter. For example, some massive particles such as the W and Z bosons are not made of quarks and leptons. This definition of matter leads to "two groups" of particles, matter (quarks and leptons) and force carriers (gauge bosons).