Matter is typically defined as any thing that has mass and occupies a volume of space.
At a fundamental level, everything from leptons and quarks, to entire atoms and molecules are considered matter. And, obviously, anything made with these constituents also constitutes matter.
However, this definition is usually only extended to normal matter. In astrophysics and cosmology another form of matter is also discussed, specifically dark matter.
Normal Matter
Normal matter is the luminous matter that we see all around us. It is composed of leptons (electrons for example) and quarks (the building blocks of protons and neutrons), which can be used to build atoms and molecules which, in turn, are the lattice work of everything from humans to stars.
Normal matter is said to be luminous not because it necessarily "shines" but because it interacts electromagnetically, as well as gravitationally, with other matter and radiation.
Another aspect of normal matter is antimatter. All particles have an anti-particle that has the same mass but opposite spin and charge (and color charge when applicable). When matter and antimatter collide the annihilate and create pure energy in the form of gamma-rays.
Dark Matter
In contrast with normal matter, dark matter is that which is non-luminous. That is, it does not interact electromagnetically and therefore it appears dark (i.e. it will not reflect light). The exact nature of dark matter is not well known.
Currently there are three basic theories for the exact nature of dark matter:
- Cold Dark Matter (CDM): While considered the best candidate to explain dark matter, there isn't a strong candidate particle that has been measured. Generically called the Weakly Interacting Massive Particle (WIMP) there is a general lack of justification for existence for such particles; namely we are not certain how they would arise under natural circumstance. Other possibilities for CDM include Axions - theoretical particles needed to explain certain phenomenon in Quantum Chromodynamics (QCD). Though these particles also have never been detected. And, finally, MACHOs (MAssive Compact Halo Objects) could explain the mass, but the specific dynamics remain a reach. These objects would include black holes, ancient neutron stars and planetary objects which are all non-luminous (or nearly so) but still contain a significant amount of mass. The problem here is that there would have to be a lot of them (more than would be expected given the age of certain galaxies) and their distribution would have to be surprisingly (impossibly?) uniform.
- Warm Dark Matter (WDM): This form of dark matter is thought to be composed of sterile neutrinos. These are particles that are similar to normal neutrinos save for the fact that they are much more massive and do not interact via the weak force. Another candidate for WDM is the gravitino. This is a theoretical particle that would exist should the theory of Supergravity - a blending of general relativity and supersymmetry - gain traction. Certainly, evidence for the existence of a gravitino would be significant for both realms of physics. WDM is also an attractive candidate, but the existence of either sterile neutrinos or gravitinos is speculative at best.
- Hot Dark Matter (HDM): The subset of particles considered to be Hot Dark Matter are the only ones already known to exist: Neutrinos. The problem with this explanation is that neutrinos travel at nearly the speed of light and therefore would not "clump" together in ways that we project dark matter would. Also given that the neutrino is nearly massless (it actually was thought to be massless for a long time, but was finally determined to have mass due to the ability of the particle to change "flavors" spontaneously) an incredible amount of them would be needed to meet the needed deficit. One explanation is that there is a yet-undetected type or flavor of neutrino that would be similar to those already known to exist except would have a significantly larger mass (and hence perhaps slower speed). But this is would probably be more similar to Warm Dark Matter.
Matter-Radiation Connection
According to Einstein's theory of relativity, mass and energy are equivalent. As such with enough radiation (light) collides with other photons (another word for light "particles") of sufficiently high energy, mass can be created.
The typical process for this is a gamma-ray collides with matter of some sort (or another gamma-ray) and the gamma-ray will "pair-produce" creating an electron-position pair. (A positron is the anti-matter particle of the electron.)
So while radiation is not explicitly considered matter (it does not have mass or occupy volume, at least not in a well defined way), it is vitally connected to matter in that radiation crates matter and matter creates radiation (like when matter and anti-matter collide).
Dark Energy
Taking the matter-radiation connection a step further, theorists also propose that a mysterious radiation exists in our Universe known as dark energy. The nature of dark energy is not understood at all. But it is hoped that as the secrets of dark matter are uncovered that we will come to understand the nature of dark energy as well.
