Before we talk about dark matter, we need to talk about gravity.
Gravity pulls things together. It keeps the Earth orbiting around the Sun. It keeps you stuck to the Earth. The universe is full of matter and the attractive force of gravity pulls all matter together.
The more massive an object is, the greater it's gravitational pull. In other words, you need to have a big mass to exert a big gravitational pull.
Now, let's looks at the above picture of our Milky Way galaxy. You can see the area where our Sun lives, nestled on the Orion Spur.
A galaxy is just a collection of stars, dust and other space matter that's usually circling around a massive black hole.
Everything is held together by gravity into one spinning mass. The more matter a body has, the more massive its gravitational field. And the Milky Way looks pretty massive, doesn't it?
But, there isn't enough visible matter in our galaxy for it to exist.
Our galaxy is rotating with such speed that the gravity generated by the all the stars, dust and other space matter you can see could not possibly hold it together. Our galaxy should have torn itself apart long ago.
That's right. Our galaxy isn't fat enough.
And it's not just our galaxy. There isn't enough matter to hold together other galaxies in our universe.
This is where dark matter comes in.
The mystery of the missing matter
Dark matter does not emit light or energy - so it's impossible for scientists to directly observe it. But we can detect dark matter through its gravitational effects.
And here's a sobering thought - roughly 80% of the matter in our universe is made up for dark matter. Only 20% is the stuff we can actually see.
But what is dark matter? Well, no one really knows because we can't directly detect it. That's right. We have no idea what 80% of the matter in our universe actually is.
One idea is that it could contain "super-symmetric particles" - which is where every particle has a more elusive partner. There are hopes that CERN's Large Hadron Collider could detect super-symmetric particles. Another idea is that dark matter may be a special version of a particle called a pion. And most proposals assume dark matter is made up of a special type of matter that doesn't interact with itself very much - these particles are called WIMPs (Weakly Interacting Massive Particles).
There is another theory - that the laws of gravity need revising. Or there could be a mysterious "dark energy" that overcomes gravity.
We'll look at dark energy next week!
Extra reading and watching
If you want to find out more about CERN and its efforts to detect dark matter - click here. And this video gives a great overview of what we actually know about dark matter (with a little more info on dark energy):
If you've got an hour to spare, this video from the World Science Festival gives a really in-depth explanation of dark matter:
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