3.2.8 Dark matter

What do scientists think holds galaxies together?

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DAVID MITCHELL:

60-second Adventures in Astronomy, number eight, Dark Matter. What’s the matter with dark matter?

Fritz Zwicky was a Swiss astronomer who could probably get you 81 points on a triple word score in Scrabble. In the 1930s he noticed that galaxies within clusters were zooming around far quicker than their mass would logically dictate. So he figured that there must be some extra mass in there- - some sort of dark, invisible matter slurping around the universe. He imaginatively called this dark matter dark matter.

But the problem is trying to prove it. Because unlike other dark things, you can see right through this stuff. And this gave Zwicky another idea. According to Einstein’s theory of general relativity, the more mass something has, the more it magnifies and distorts objects that you can see through it.

So by studying the distortion of distant galaxies, we can calculate that there must be some extra mass between us and them. But because we can’t see it, touch it, or weigh it, it’s not surprising that we can’t figure out exactly what it is. And that’s what the matter with dark matter is. It makes up most of the mass in the universe, but when it comes to knowing the details, we’re still in the dark.

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Many spiral galaxies have had their masses estimated from how fast their stars are orbiting. The more mass that a galaxy contains, the faster its stars go in their orbits. This is because the more powerful the tug of gravity, the faster the stars have to move to avoid falling towards the centre.

The trouble is, if you add up all the masses of the stars in a galaxy, plus the mass of gas, it doesn’t come close to the mass estimated from the stars’ orbits. If stars and gas were all there was in a galaxy, the stars would be flung out of the galaxy, because they are moving so quickly. So what’s keeping galaxies together? It must be something non-luminous because it cannot be seen. Astronomers have called this dark matter.

Dark matter is needed to explain the motion of galaxies in general and in galaxy clusters in particular. Dark matter is found wherever ‘normal’ matter, such as the stuff that makes up galaxies (of all shapes), is found. For example, a large galaxy cluster will contain a very great amount of dark matter, which exists within and around the galaxies that make up that cluster.

Dark matter is so named because astronomers don’t know what it actually is – we can’t see it directly and can only infer its existence by the effect it has on the regular matter that we can see.

We are not able to observe dark matter directly, however the strong gravity can distort the light from galaxies behind the dark matter on its way to us. This distortion is known as ‘gravitational lensing’ and is a useful way of observing black holes and dark matter. Read more about gravitational lensing from the link below.

Next, we’ll look at dark energy.