What is Matter?

dark matter blobs
This Hyper Suprime-Cam image shows a small (14 arc minute by 9.5 arc minute) section of galaxy clusters with the outlines of a dark matter concentration and part of another traced out with contour lines. The stars and galaxies are made up of regular, "luminous" matter. Subaru Telescope/National Astronomical Observatory of Japan

Matter is All Around Us

We seldom stop to think about it as we go about our daily lives, but we are matter.  Everything we detect in the universe is matter. It's the fundamental building block of everything: you, me and all the life on Earth, the planet we live on, the stars, and galaxies. It's typically defined as anything that has mass and occupies a volume of space. 

We're made up of atoms and molecules, which are also matter. The definition of matter is anything that has mass and takes up space. This includes normal matter as well as dark matter

However, that definition is reall only extended to normal matter. Things change when we get to dark matter. Let's talk about the matter we CAN see, first. 

Normal Matter

Normal matter is the matter that we see all around us.  It's often referred to as "baryonic matter" and is made 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 luminous, not because it "shines", but because it interacts electromagnetically and  gravitationally with other matter and with 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 matter that is non-luminous. That is, it does not interact electromagnetically and therefore it appears dark (i.e. it will not reflect or give off 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):  There is one candidate called the weakly interacting massive particle (WIMP)  that could be the basis for cold dark matter. However, we don't know much about it or how it would arise. Other possibilities for CDM include axions, however they have never been detected. Finally, there are MACHOs (MAssive compact halo objects), They could explain the measured mass of dark matter. These objects 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. However, there's a problem. 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 to explain the dark matter that astronomers have found "out there".
  • 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. WDM is also an attractive candidate to explain dark matter, but the existence of either sterile neutrinos or gravitinos is speculative at best.
  • Hot dark matter (HDM): The particles considered to be hot dark matter already exist. They're called "neutrinos". They travel at nearly the speed of light and don't "clump" together in ways that we project dark matter would. Also given that the neutrino is nearly massless, an incredible amount of them would be needed to make up the amount of dark matter that exists. One explanation is that there is a yet-undetected type or flavor of neutrino that would be similar to those already known to exist. However, it would have a significantly larger mass (and hence perhaps slower speed). But this is would probably be more similar to warm dark matter.

The Connection between Matter and Radiation

According to Einstein's theory of relativity, mass and energy are equivalent. If 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". This creates 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 connected to matter. This is because radiation creates 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. It's called dark energy. The nature of this mysterious radiation is not understood at all. Perhaps when dark matter is understood, we will come to understand the nature of dark energy as well.

Edited and updated by Carolyn Collins Petersen.