WIMPS: The Solution to the Dark Matter Mystery?

Weakly Interacting Massive Particles

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

There's a big problem in the universe: there is more mass in the galaxies than we can account for by simply measuring their stars and nebulae. It seems to be true of all galaxies and even the space between galaxies. So, what is this mysterious "stuff" that seems to be there, but can't be "observed" by conventional means? Astronomers know the answer: dark matter.  However, that doesn't tell them what it is or what role this dark matter has played throughout the history of the universe. It remains one of the great mysteries of astronomy, but it won't remain mysterious for long. One idea is the WIMP, but before we can talk about what it might be, we need to understand why the idea of dark matter even came up in astronomy research. 

Finding Dark Matter

How did astronomers even know dark matter was out there? The dark matter "problem" began when astronomer Vera Rubin and her colleagues were analyzing galactic rotation curves. Galaxies, and all the material they contain, rotate over long periods of time. Our own Milky Way Galaxy rotates once every 220 million years. However, not all parts of the galaxy rotate the same speed. Material closer to the center rotates faster than material in the outskirts. This is often referred to as "Keplerian" rotation, after one of the laws of motion devised by astronomer Johannes Kepler. He used it to explain why the outer planets of our solar system seemed to take longer to go around the Sun than the inner worlds do. 

Astronomers can use the same laws to determine galactic rotation rates and then create data charts called "rotation curves". If galaxies followed Kepler's Laws, then the stars and other light-emitting objects in the inner part of the galaxy should rotate around more rapidly than the material in the outer parts of the galaxy. But, as Rubin and others found out, the galaxies didn't quite follow the law.  

What they found was vexing: there wasn't enough "normal" mass — stars and gas and dust clouds — to explain why the galaxies weren't rotating the way the astronomers expected. This presented a problem, either our understanding of gravity was seriously flawed, or there was about five times more mass in the galaxies that astronomers couldn't see.

This missing mass was dubbed dark matter and astronomers have detected evidence of this "stuff" in and around galaxies. However, they still don't know what it is. 

Properties of Dark Matter

Here's what astronomers DO know about dark matter. First, it doesn't interact electromagnetically. In other words, it can't absorb, reflect or otherwise mess with light. (It can bend the light due to the gravitational force, however.) In addition, dark matter has to have some significant amount of mass. This is for two reasons: the first is that dark matter makes up a lot of the universe, so a lot is needed. Also, dark matter clumps together. If it really didn't have a lot of mass, it would move close to the speed of light and the particles would spread out too much. It has a gravitational effect on other matter as well as light, which means it has mass.

Dark matter does not interact with what's called the "strong force". This is what binds the elementary particles of atoms together (starting with quarks, which bond together to make protons and neutrons). If dark matter does interact with the strong force, it does so very weakly.

More Ideas about Dark Matter

There are two other characteristics that scientists think dark matter has, but they are still debated pretty heavily among theorists. The first is that dark matter is self-annihilating. Some models contend that particles of dark matter would be their own anti-particle. So when they meet other dark matter particles they convert into pure energy in the form of gamma rays. Searches for gamma-ray signatures from dark matter regions have not revealed such a signature however. But even if it was there, it would be very weak.

In addition, candidate particles should interact with the weak force. This is the force of nature that is responsible for decay (what happens when radioactive elements break down). Some models of dark matter require this, while others, like the sterile neutrino model (a form of warm dark matter), argue that dark matter would not interact in this way.

The Weakly Interacting Massive Particle

Okay, all of this explanation brings us to what dark matter could possibly BE. That's where the Weakly Interacting Massive Particle (WIMP) comes into play. Unfortunately, it's also somewhat mysterious, although physicists are working to know more about it.  This is a theoretical particle that meets all of the above criteria (though may or may not be its own anti-particle). Essentially, it's a kind of particle that began as a theoretical idea but is now being researched using superconducting supercolliders such as CERN in Switzerland.

The WIMP is classified as cold dark matter  because (if it exists) it is massive and slow. While astronomers have yet to directly detect a WIMP, it's one of the prime candidates for dark matter. Once WIMPs are detected astronomers will have to explain how they formed in the early universe. As is often the case with physics and cosmology, the answer to one question inevitably leads to a whole host of new questions.

Edited and updated by Carolyn Collins Petersen.