Imagine you had a big box of dark matter. What would it look like?
You're probably imagining a big black cube, aren't you? Actually, it would be perfectly transparent. That's because it's not really "dark" at all.
OK, it doesn't emit light, but it also doesn't absorb light either, or we'd be able to see it as shadows against the background.
You also can't touch it. If you tried to scoop up a handful of dark matter, it would pass right through your hand. The only way we can tell dark matter is there is from the gravitational pull it has.
Right now, dark matter is streaming through your body. Our Sun and all the solar system is travelling through our galaxy, with the dark matter wafting past us and through us.
Now, we can't see dark matter with our eyes, or touch it with our fingers, but sometimes a dark matter particle will still manage to collide with a particle of ordinary matter.
These collisions are very rare, but scientists have made dark matter detectors to look for these collisions as the dark matter streams through. It's a bit like holding your hand out of a window of a moving car and feeling the breeze.
A lot of the evidence for dark matter is that the visible matter seems to be moving too quickly, like the spinning of spiral galaxies, or the movement of galaxies in a galaxy cluster. There has to be some unseen matter tugging at the visible matter, to explain how quickly it's moving.
In fact, the discrepancy is so big that most of the matter in the Universe has to be dark matter! Everything you see around you is just a tiny fraction of what's really there.
Not only is dark matter wafting through you right now, but you're also warping the space around you.
According to Einstein, every object causes some curvature in the space around it. The curvature from a person is too small to measure directly, but galaxies and clusters of galaxies cause so much warping that background things look distorted.
From that distortion we can figure out how much matter there is - which is another line of evidence for dark matter.
Some scientists have argued that Einstein got it wrong about gravity, and that what we're calling evidence for dark matter is just a sign that we've got gravitational tugs from the visible matter wrong.
But an image of two galaxy clusters in a middle of a collision have made the case for dark matter very strong. The Hubble Space Telescope took a picture, and from the distorted background galaxies, the scientists figured out where the matter was. In the picture, this is shaded in blue.
Meanwhile, the Chandra X-ray space telescope (there are lots of space telescopes!) took a picture and found out where the gas is. This is red in the picture. Now, when gas collides with gas, you get all sorts of messy turbulence and mixing and maybe shock waves. But when dark matter meets dark matter, it just passes right through.
So, even if your hand was made of dark matter, you still wouldn't be able to scoop up a handful!
What the astronomers saw in the galaxy cluster collision is that the gas (red) got stuck in the middle, while most of the matter (blue) passed right through and out the other side. This is very hard to explain in any way, unless most of the matter is dark matter.
Find out more
Find out more about the photo above: 1E 0657-56 - NASA finds direct proof of dark matter
You can't touch dark matter, but you can get to know it - and other fundamental parts of our universe - by studying with The Open University: How the Universe works
From LearningSpace from OpenLearn: What contributes to the spectra of galaxies?