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

We are surrounded by matter. In fact, we ARE matter. Everything we detect in the universe is also matter. It's so fundamental that we simply accept that everything is made of matter. It's the fundamental building block of everything: 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.

The building blocks of matter are called "atoms" and "molecules." They, too, are matter. The matter we can detect normally is called "baryonic" matter. However, there is another type of matter out there, which can't be directly detected. But its influence can. It's called dark matter

Normal Matter

It's easy to study normal matter or "baryonic matter". It can be broken down into sub-atomic particles called leptons (electrons for example) and quarks (the building blocks of protons and neutrons). These are what make up the atoms and molecules which are the components of everything from humans to stars.

Illustration of an atomic nucleus as a series of red and white circles, orbited by electrons represented by white circles.
Computer illustration of an atomic model containing atoms, protons, neutrons, and electrons. These are the building blocks of normal matter. Science Photo Library/Getty Images

Normal matter is luminous, that is, it interacts electromagnetically and gravitationally with other matter and with radiation. It doesn't necessarily shine like we think of a star shining. It may give off other radiation (such as infrared).

Another aspect that comes up when matter is discussed is something called antimatter. Think of it as the reverse of normal matter (or perhaps a mirror-image) of it. We often hear about it when scientists talk about matter/anti-matter reactions as power sources. The basic idea behind antimatter is that all particles have an anti-particle that has the same mass but opposite spin and charge. When matter and antimatter collide, they annihilate each other and create pure energy in the form of gamma rays. That creation of energy, if it could be harnessed, would provide huge amounts of power for any civilization that could figure out how to do it safely.

Dark Matter

In contrast with normal matter, dark matter is material 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. However, its presence can be detected by the gravitational effect it has on normal matter. For example, its presence can constrain the motions of stars in a galaxy, for example.

dark matter blobs
Dark matter in the universe. Could it be made of WIMPs? This Hyper Suprime-Cam image shows a small (14 arc minute by 9.5 arc minute) section of galaxy clusters with the outlines of one dark matter concentration and part of another traced out with contour lines. Subaru Telescope/National Astronomical Observatory of Japan

Currently there are three basic possibilities for "things" that make up 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, scientists don't know much about it or how it could have been formed early in the history of the universe. Other possibilities for CDM particles 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. Those would conveniently explain dark matter, but 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 incredibly well spread out throughout the universe to explain the dark matter that astronomers have found "out there." So, cold dark matter remains a "work in progress."
  • Warm dark matter (WDM): This 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 is known to exist. 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 would probably be more similar to warm dark matter.

    The Connection between Matter and Radiation

    Matter doesn't exactly exist without influence in the universe and there's a curious connection between radiation and matter. That connection wasn't well understood until the beginning of the 20th century. That's when Albert Einstein began thinking about the connection between matter and energy and radiation. Here's what he came up with: according to his 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. This process is what scientists study in giant laboratories with particle colliders. Their work delves deeply into the heart of matter, seeking the smallest particles that are known to exist.

    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. Its nature 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.