Stephen Cornish - Facebook asked:
Dark matter is invisible to the naked eye but has no one exhausted using different ways of viewing, such as when one uses infra-red, ultraviolet light or spectrogram to examine things the naked eye can't see?
We put Stephen's question to dark matter expert Andrew Pontzen:
Andrew - Dark matter is this extra stuff that we think has to be in the universe. We really can't make sense of what we do see out there in the universe without supposing there is some extra stuff that we can't see. The reason that we think it’s got to be there is by watching the way that stuff moves around perhaps most famously, the way that stars move around inside galaxies. They kind of whirl around inside their galaxies really fast. That tells us that something has to be making them whirl around that fast. We think it’s the pull of gravity but the pull of gravity just wouldn’t be strong enough from the stuff that you can actually see directly. So, we suppose it must be some extra stuff there that we just can't see.
Kat - This is basically the stuff that makes the maths of the universe work. This must be something balancing it up and you seem that’s dark matter.
Andrew - Exactly, yeah. Galaxies is just one example I should say. There are load of examples where this extra stuff – you know, about 5 times more stuff than we can actually see. It just makes everything balance up much better.
Kat - So, why can't we see it? I mean clearly, it’s dark. So, how would you see it?
Andrew - People have tried in many different ways. And so, the question I think was asking…
Kat - Have we exhausted all the ways of doing it?
Andrew - Yeah and I suppose the short answer is no, we haven't actually exhausted all the ways. We’ve looked pretty damn hard though. We’ve looked in many, many different types of light. So you have things like infrared light, microwave light, you have ultraviolet light. We have all these different types of lights. We have looked into all of those to see if we can see it and the answer is pretty uniformly no, but if our theories about what is dark matter are correct and we do have some ideas about what it might actually be then perhaps it should actually turn up. Occasionally, you might be able to see it in the form of gamma rays. So, gamma rays are very, very high energy type of light and that’s where we think the best place to look is, and people have been trying to do this experiment. There are gamma ray telescopes which you can look into space with and they particularly look at the centre of our galaxy where we think there's a big concentration of dark matter.
Chris - Weren’t people building a big dark matter detector deep underground though Andrew? The rationale being that if you build it along way underground in say an old goldmine kilometres down, the overlying rock is going to filter out lots and lots of radiation that might fool you into thinking you're detecting dark matter, making it easier to spot?
Andrew - That’s absolutely correct. That’s what we called direct dark matter detection. So, that’s trying to find the actual particles, the bits and pieces that actually make up dark matter as they come through the Earth. But when we look in gamma rays, we’re trying to see what we call dark matter annihilation. So, these particles actually colliding with each other and turning into energy in the form of light, this very high energy light, and we’re looking to see that using these gamma ray telescopes.
Kat - But presumably, to answer our questioner, no one has seen it yet.
Andrew - Nobody has seen it yet. Some people say, maybe we’ve seen hints, but there's certainly nothing that everyone agrees on.