Understanding the nature of dark matter is one of the most important unsolved problems in physics. Maybe you’ve heard about it in the news. I’ve found personally that the general public is completely unaware of what it is. When I learned about the Science Blog, it struck me as a great opportunity to bring this topic to a wider audience.
The story of dark matter began in 1933, when a physicist named Fritz Zwicky was looking at a galaxy cluster. A galaxy cluster is simply a group of many galaxies bound together by gravity. In the same way that gravity causes planets to rotate around the sun, it causes these galaxies to rotate around a point somewhere in the middle of the cluster. Incidentally, that point is called the centre of mass. As the total mass of a cluster increases, the galaxies are more tightly bound, and they rotate faster.
Zwicky could estimate the mass of the entire cluster and use that information to predict how fast the galaxies should be orbiting. He observed, however, that they were moving much faster than he expected. At the rotation speed Zwicky observed, the cluster should have been ripped apart! It would take a much greater mass to hold it together.
What are we to make of this?
Zwicky reasoned that there must be some additional mass in the cluster that he couldn’t see, behind the high rotational speeds . He coined the term “dark matter” to refer to this extra mass. For about 50 years, the existence of dark matter was controversial. But now, we are confident it exists, thanks to an enormous body of evidence. All of these sources of evidence suggest that there is an enormous amount of matter in the universe that is invisible to us. If you’d like to read more about the evidence for dark matter—I don’t have room to write about it here—you can look up Vera Rubin’s research, galactic rotation curves, Cosmic Microwave Background anisotropy or the Bullet Cluster.
But, despite all of this evidence, we still have almost no idea what dark matter is.
If you asked me what ordinary matter—things like me, you, the earth or the sun—is made of, I could rattle off a list of fundamental particles, along with their properties, and I could tell you precisely how they interact. I can’t do the same for dark matter. At best, I can give you a few facts that we’ve figured out by studying the body of research that is out there.
First, dark matter is abundant: by mass, there’s about five times more dark matter than visible matter in the universe. There’s dark matter passing through you as you read this article; you just don’t notice it because it hardly interacts with you.
Next, there are four known forces through which particles may interact with others: gravity, electromagnetism, strong nuclear force, and weak nuclear force. Dark matter particles must interact with regular matter through gravity. It is possible that they also interact through weak nuclear force. They cannot participate in the electromagnetic or strong nuclear interactions—if they did, we would easily detect them.
Finally, dark matter is not made up of the same particles that constitute ordinary matter. These are new particles that we have not yet discovered.
This is all horribly vague, as you’ve likely noticed. But I have to be vague, since we know so little about dark matter. Determining the nature of dark matter poses a great challenge for physics. Theoretical physicists have been constructing models of dark matter particles, and experimental physicists have been trying to detect them. Researchers at Carleton have been active participants in this effort. One of our experimentalists, Mark Boulay, was recently in the news after he received funding to build a dark matter detector. Unfortunately, no experiment to date has directly observed dark matter.
No one knows how long it will be before we understand the nature of dark matter. It could be this year or it could be decades from now. All we can do is keep searching for it.
Graphic by Manoj Thayalan
