What is dark matter?

A primer on dark matter - history of observations, and what we know, with Astronomer Royal Martin Rees.
14 May 2019

Interview with 

Professor Lord Martin Rees, Cambridge University


A large astronomical telescope against a dark starry sky.


We’re going to be delving into the mysterious stuff that makes up a massive amount of the Universe. But we can’t see it, and we’ve haven’t the foggiest idea what it is. So how are we trying to actually find out, and how do we even know it’s there at all? Ben McAllister has been finding out...

Ben - I'd like to tell you a story. It's a story about galaxies, black holes, stars, planets, people, and everything else in the universe. We now know that all of the big stuff in the universe - people, planets, stars - is made up of a handful of different kinds of particles. These tiny little things like atoms which are made up of protons, neutrons and electrons. We've come to know quite a lot about those little things; how they make up bigger things over the last few hundred years. Collectively, astronomers call all that stuff baryonic matter, and that's kind of all there is, right?

I'm here to tell you that that's not the case. The baryonic matter - people, planets, stars - is only a very small fraction of the whole story. In today's program we're going to hear what we know about the rest of it. We're going to hear about dark matter.

To give you a taste, dark matter is this mysterious stuff that occupies the universe. It's enormous, there's five times as much of it as there is regular matter, it's everywhere. Right now, as you listen it's passing right through your body; we can't see touch or feel it. But before we get to what it is, we have to go back a bit. Have you ever looked at the stars and wondered if there's more out there? If you have, you really aren't alone, humans have been doing it for as long as there gave been humans.

Professor Lord Martin Rees, Astronomer Royal...

Martin - It was actually some of which emerged in the 1930s through the work of Fritz Zwicky, who was a Swiss-American astronomer; and he was studying the distribution of galaxies. Each galaxy is of course as big as our Milky Way, so he was looking at the universe on very large scales. He realised that the galaxies weren't distributed randomly, but they were clusters, and these clusters obviously seem to be held together by gravity. But when he measured the speeds of these galaxies he found it was surprising that they weren't flying apart, because the energy corresponding to those speeds would overwhelm the gravitational force holding the cluster together if that gravity was just due to the galaxies. He inferred that there must be some extra material that bound the cluster together, and this was the first really serious evidence that there was some dark stuff in the universe over and above the gas and stars that are visible.

Ben - For decades we've been observing things like this. Strange movements of large bodies in space that can't be explained if we only consider the matter that we can see. It all comes down to gravity. Gravity is the main force that governs the way things move around in space. It's a force that exists between any two things that have mass and it pulls them together. Gravity gets stronger the more mass there is but importantly, it gets weaker the further apart the two things are.

In space, when we look at the stuff we can see like stars, for example, we can estimate how much mass there is in the system; and then, by using the laws of gravity - what we call Newtonian gravity - we can model the way we expect the mass to move. When things don't move the way we expect, say they move much faster for example, it implies that something's missing from our picture. There's some extra force making things move around faster which points to their being some extra mass to provide that extra force...

Martin - If you had found, for instance, that Jupiter was going round the Sun as fast as the Earth was, you'd have had to infer that there was a lot of mysterious mass outside the Earth's orbit, but inside Jupiter's orbit. So Jupiter was feeling not just the mass of the Sun, but something extra which the Earth wasn't feeling. Something like that, on a far bigger scale of course, happened when people studied the outer parts of galaxies. They found that the material was going round faster, the outlying stars and the gas at large distances was going faster, and this implied that the stars in a galaxy were not the dominant kind of mass, and that a whole, this galaxy like ours was embedded in what came to be called a halo of some material which was not emitting any light but was exerting a strong gravity, and was dominating the gravitational pull in the outer parts of the galaxy.

Ben - We've arrived at the point in the story where, thanks to observations of bodies moving around in space, we're pretty sure we're surrounded by massive amount of dark matter. Again, it's moving through your body right now, and it massively outweighs the regular matter that we understand, we just don't know what it is. We've since figured out a little more about it, but not that much more. It's a new frontier, a new region to explore. We do have some theories to explain the phenomena we see, some of which don't actually include any dark matter at all...

Martin - And there is, of course, the idea that we are wrong about gravity. And of course, all the arguments where you infer a mass from the motions of planets and stars and galaxies: that is assuming, in a sense, Newtonian gravity. So some people are proposing other ways where we wouldn't need to have dark matter at all, and we would simply have a different theory of gravity. But I think most people are against that, because first of all there's no particular reason why we should be surprised by dark matter. There's lots of scope for dark matter particles. And secondly we'd be jettisoning a lot of good data if we abandon the idea that we understood gravity. I would still bet that it's most likely that dark matter is in some kind of particles.

Ben - A number of experiments around the world propose to try and detect these particles as they pass through the earth and we'll hear more about some of those later. But why should we care about this? We can't see, touch or feel it, it's just this mysterious stuff that floats on by...

Martin - Well, we know everyone has gazed up throughout human history at the stars and wondered about them. One of the great achievements of cosmology is to understand the structure of the universe - why there are stars, why there are galaxies, why they are clustered, and the details of that. This is only giving us a consistent story if we have the presence of dark matter which is, on average in the universe, five times as dense as the gas and stars that we see. And I think this success is one of the great achievements of modern science. I would say it's up there with the standard model of particle physics in the genome. When the history of science is written I think the fact that we understand cosmic evolution, and why galaxies exist, is really a very great achievement.

Ben - If that doesn't do it for you, consider this. Think about everything humans have accomplished with an understanding of just one sixth of the matter in the universe. Computers. Modern medicine and spaceflight. All of art literature. Imagine what we could do if we could unlock the rest.


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