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Dark matter 101


Regular matter

In this universe, being made of regular matter makes stuff like us really weird. (Source: AnnaRise/iStockPhoto)

Over the last four centuries since Galileo fired up his telescope and went looking at the night sky, we have discovered so much. But, as we uncover more knowledge, we also uncover further mysteries.

For example, we now know that only about 4 or 5 per cent of the universe is made from normal matter. So you and me, and the stars and the planets, are made from regular matter based on atoms — that is, made up of protons, neutrons and electrons. (Just as an aside, the mass of all the hot gas between the galaxies is much much greater than the mass of all the stars inside all the galaxies.)

About 73 per cent of the universe is made up of so-called dark energy. The 2011 Nobel Prize for Physics was awarded for this discovery. (Don't worry, I'm not going to even begin talking about dark energy — just yet).

And then there's a large chunk of the universe, about 22 per cent, made from dark matter. Dark matter is not made from atoms — the stuff that people and plants, stars and gas are made of. It's something strange and unknown that we currently don't understand. We cannot see this mysterious missing mass of the universe, the dark matter, because it does not radiate or interact with light — in fact, it doesn't seem to radiate or interact with any kind of electromagnetic radiation at all.

So just to get a sense of where we stand in the big picture, our universe is dominated by dark energy and dark matter. You and I are made of stuff that, in percentage terms, is almost just a rounding error.

In one sense, dark matter is like the wind — we can't see the wind directly, but we can see the effect of what the wind does. So when we see the leaves on trees fluttering and the branches bending, we know that there's some wind, even if we can't see it.

In the case of dark matter, it doesn't interact with electromagnetic radiation, but it does interact with gravity. Because dark matter has mass, it "makes" its own gravity, and in return, dark matter is affected by the gravity from other objects. So dark matter pulls on both regular and dark matter. And the opposite happens — it can be attracted by the gravity of both regular matter and other dark matter.

As a result of its gravity, dark matter has another strange property. It's the invisible scaffolding that is responsible for the formation of the stars and galaxies. That's right, dark matter helped make our current universe.

After the Big Bang, the universe was mostly dark and smooth for about 100 million years. There was dark matter back then, probably making up about 63 per cent of the mass of the universe. This dark matter had mass, and so it had gravity, and so it began to coalesce under its own gravity, and began to form structures from the dark matter. The gravity of dark matter attracted regular matter like hydrogen, helium and lithium, which condensed and formed into stars. These stars then formed into galaxies.

So let's take a little detour into our own galaxy, the Milky Way, before we come back to dark matter.

It's called the Milky Way because it looks like a milky band stretching across the night sky, and because all you can see with the naked eye is the fuzzy milky glow. You really need binoculars or a telescope to pick out the individual stars — and there are a few hundred billion of them in our Milky Way. It was only in the 1920s that we realised that it was just one of many galaxies in our universe. Today we reckon that there are a few hundred billion galaxies.

Beginning in the 1970s and 1980s, our astronomers began to develop new types of telescopes — both on the ground and in space. They didn't look only at the visible Milky Way, but also at the electromagnetic radiation it was emitting between microwaves and x-rays — and suddenly, we discovered a whole lot more about our galaxy. By around the year 2000, the astronomers had set up systematic mapping programs. They began to find huge structures sprawling across the sky, which were so big that we had simply never noticed them before.

So I'll throw more light on our Milky Way, and talk about dark matter, next time...

Tags: the-universe, energy, stars, galaxies

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Published 06 March 2013

© 2014 Karl S. Kruszelnicki Pty Ltd