Cold Dark Matter: the Mysterious Unseen Stuff of the Universe

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 one dark matter concentration and part of another traced out with contour lines. Subaru Telescope/National Astronomical Observatory of Japan

The universe is made up of at least two kinds of matter. Primarily, there's the material we can detect, which astronomers call "baryonic" matter. It's thought of as "ordinary" matter because it's made of protons and neutrons, which can be measured. Baryonic matter includes stars and galaxies, plus all the objects they contain.

However, there is also "stuff" out there in the universe that can't be detected through normal observational means. Yet, it does exist because astronomers can measure its gravitational effect on baryonic matter. Astronomers call this material "dark matter" because, well, it's dark. It doesn't reflect or emit light. This mysterious form of matter presents some major challenges to understanding a great many things about the universe, going right back to the beginning, some 13.7 billion years ago. 

The Discovery of Dark Matter

Decades ago, astronomers found that there wasn't enough mass in the universe to explain things like the rotation of stars in galaxies and the movements of star clusters. Mass affects an object's motion through space, whether it's a galaxy or a star or a planet. Judging by the way some galaxies rotated, for example, it appeared that there was more mass out there somewhere. But, it wasn't being detected. It was somehow "missing" from the mass inventory they assembled using stars and nebulae to assign a galaxy a given mass. Dr. Vera Rubin and her team were observing galaxies when they first noticed a difference between expected rotation rates (based on estimated masses of those galaxies) and the actual rates they observed.

Researchers began to dig more deeply into figuring out where all the missing mass had gone. They considered that perhaps our understanding of physics, i.e. general relativity, was flawed, but too many other things didn't add up. So, they decided that perhaps the mass was still there, but simply not visible.

While it is still possible that we are missing something fundamental in our theories of gravity, the second option has been more palatable to physicists. Out of that revelation was born the idea of dark matter. There's observational evidence for it around galaxies, and theories and models point to the involvement of dark matter early in the universe's formation. So, astronomers and cosmologists know it's out there, but haven't yet figured out what it is yet.

Cold Dark Matter (CDM)

So, what could dark matter be? As of yet, there are only theories and models. They can actually be slotted into three general groups: hot dark matter (HDM), warm dark matter (WDM), and cold dark matter (CDM).

Of the three, CDM has long been the leading candidate for what this missing mass in the universe is. However, some researchers still favor a combination theory, where aspects of all three types of dark matter exist together to make up the total missing mass.

CDM is a kind of dark matter that, if it exists, moves slowly compared to the speed of light. It is thought to have been present in the universe since the very beginning and has very likely influenced the growth and evolution of galaxies. as well as the formation of the first stars. Astronomers and physicists think that it's most likely some exotic particle that hasn't yet been detected. It very likely has some very specific properties:

It would have to lack an interaction with the electromagnetic force. This is fairly obvious, since dark matter is dark. Therefore it doesn't interact with, reflect, or radiate any type of energy in the electromagnetic spectrum. 

However, any candidate particle that makes up cold dark matter would have to take into account that it has to interact with a gravitational field. For proof of this, astronomers have noticed that dark matter accumulations in galaxy clusters wield a gravitational influence on light from more distant objects that happens to be passing by. This so-called "gravitational lensing effect" has been observed many times.

Candidate Cold Dark Matter Objects

While no known matter meets all of the criteria for cold dark matter, at least three theories have been advanced to explain CDM (if they exist).

  • Weakly Interacting Massive Particles: Also known as WIMPs, these particles, by definition, meet all the needs of CDM. However, no such particle has ever been found to exist. WIMPs have become the catch all term for all cold dark matter candidates, regardless of why the particle is thought to arise. 
  • Axions: These particles possess (at least marginally) the necessary properties of dark matter, but for various reasons are probably not the answer to the question of cold dark matter..
  • MACHOs: This is an acronym for Massive Compact Halo Objects, which are objects like black holes, ancient neutron stars, brown dwarfs and planetary objects. These are all non-luminous and massive. But, because of their large sizes, both in terms of volume and mass, they would be relatively easy to detect by monitoring localized gravitational interactions. However, there are problems with the MACHO hypothesis. The observed motion of galaxies, for instance, is uniform in a way that would be hard to explain if MACHOs supplied the missing mass. Furthermore, star clusters would require a very uniform distribution of such objects within their boundaries. That seems very unlikely. Also, the sheer number of MACHOs that would have to be fairly large in order to explain the missing mass.

    Right now, the mystery of dark matter doesn't have an obvious solution — yet. Astronomers continue to design experiments to search for these elusive particles. When they do figure out what they are and how they are distributed throughout the universe, they will have unlocked another chapter in our understanding of the cosmos.