 Now we can think about what's happened in our universe. Everything appeared very hot and dense, moved apart, formed quarks, bound up into protons and neutrons. Protons and neutrons are flying around with electrons in plasma. Then finally they cool down enough to form nuclei, protons and neutrons, and then electrons join them as well to form atoms. And that's the first time we could get light because light is formed by electrons jumping between levels. So if you have electrons in a plasma and protons in a plasma, no light can travel through it. That's what's happening in the Sun. It's actually a plasma. That's why it's opaque. We can't see into the Sun. We can only see its surface. So, as atoms formed, suddenly the universe starts to go dark and atoms form. The last bit of light from that moment starts traveling off through space and we can still see it echoing around in the cosmos. And it's not for, you know, quite a long time before the gas starts to agglomerate, clump together and form dust and then form, you know, bigger lumps, discs that collapse under gravity. They all fall together and become planets and then stars and then they get hot enough because now we're starting to compress again, just certain spots around the cosmos. They're hot enough to start heating up and glowing and forming fusion and that's where all the elements that we came from. In the meantime, the universe has kept expanding, except for those little bits where there's gravity pulling it together, but, you know, the overall momentum is sending it all apart. And that brings us to today. And the question is what's going to happen in the future? Is everything just kind of keep going forever and the universe is going to just cool down and die out? That's called the heat death of the universe. Or is the gravity going to overcome the momentum and pull everything back? So it falls back into what they're calling a big crunch. So the question is how much matter is there in the universe? You could think, you could look at the stars because, you know, in our solar system the sun is huge and the rest of the planets is just a little bit of dust comparatively. But it turns out when you look at galaxies, it's all wrong. They rotate all wrong. They should spin fast in the middle where there's lots of galaxies and big black holes and stuff and lots of mass means there's lots of gravity so things have to spin quickly or they'll just collapse in together. And in the solar system, you see Pluto move very very slowly. That's what you think might happen in a galaxy. The edges move slowly, but they don't. They actually move faster than the middle. And that means there has to be a whole lot of extra matter. We can't see it. We don't know what it is. It's called dark matter. The question is, is it maybe old dead stars? Is it large planets that we can't see? Brown dwarfs? Like stars that are barely glowing, barely alive. Is it lots of black holes? They're called massive compact halo objects matter around the edges of the galaxy, which is where all this mass has to be. Or it could be there's lots of tiny little particles like neutrinos, weakly interacting, so we can't see them. They're passing through us. Massive particles. They have a bit of mass, but they're very small and there's billions of them. So they're called wimps. Turns out it's not macho. It's wimps that are dominating in the universe. Of course the physicists would say that, wouldn't they? If you tally up all the matter of the stars we can see and the dark matter, you can get an idea of how much gravity there is and whether the universe is going to come back together or whether it's going to keep going forever. And that's what Brian Schmidt decided to try and measure. Brian Schmidt is at ANU, at Mount Stromlo. These days he's actually the boss of the university, the vice chancellor. This work ended up winning Brian Schmidt a Nobel Prize along with his colleagues Saul Perlmutter and Adam Rees in 2011. As they looked at very distant supernovae, these exploding stars that are bright, but you can only just see them because they're so far away to get a feeling of how fast they're moving, what they found was, well, not that things are going to come back together. Not that they're even going to stop or drift on forever, but they're actually accelerating. And that's so weird. What is pushing it apart? We don't know. We have no idea how there can be some source of energy that can push the galaxies apart. So this is dark energy. And it turns out that's like twice as much as all the matter we have in the universe. So there's a hell of a lot of stuff now that we don't understand. All this, you know, 150 years of astronomy has basically taken us to knowing way less than we did in 1888 when Simon Newcombe thought we knew basically everything. Now, it seems like we only know about 4%.