 In this video, we'll discuss the mysterious nature of dark matter. So far, we've learned about ordinary matter, stuff like apples, cars, quad bikes, sheep, planets, and stars. Looking up at the night sky, it looks like most of the matter in the universe is stars, planets, a fair amount of intergalactic dust, and sheep. But actually, physicists are now pretty certain that everything we could possibly see and touch in the universe is only about 15% of the matter. The rest is something known as dark matter. Back in 1939, Horace Babcock was completing his PhD thesis. He was looking at spiral galaxies, which contain a central core of lots of stars and arms of stars extending outwards. These stars should be rotating about the centre of the galaxy, which they are. And like planets in the solar system, the rotation speed should decrease as you move away from the centre of the galaxy. Here's a graph of the predicted rotation speed as a function of distance from the centre of the galaxy. And here's the measured rotation speed as a function of distance. The rotational speed stays the same as you move further out, so something twice as far away will move just as quickly. But this doesn't make sense. According to Newton's law of universal gravitation, we would expect it to be decreasing in speed. If we can see all the matter, then we should be able to predict how it will all move with Newton's law of universal gravitation. So Babcock made a perfectly reasonable inference. Either some of the light from the galaxy was blocked and there was more matter than we could see, or we were fundamentally wrong about how gravity works large scales. However, there was a third possibility that Babcock either didn't think of or didn't have the audacity to suggest that our models and understandings are correct, but there's extra invisible matter that we can never hope to see, which affects the dynamics. This invisible matter is commonly called dark matter, and it's the leading contender for explaining our observations. Now if this feels like cheating to you, rest assured, most people feel that way when they first learn about dark matter, and in fact there's even an XKCD on this. But XKCD really is right. There's a huge number of strange observations at galactic scales, and dark matter explains these observations really well, to the extent that there is a near consensus among astronomers that most of the matter in the universe is dark. So what is dark matter? Well, as the name implies, it's dark. It doesn't experience electromagnetic or strong nuclear interaction, otherwise we would have seen it. That means it's entirely possible that there is dark matter streaming through the room you're sitting in right now. So why don't we walk around getting buffeted to and fro by all this dark matter everywhere? Well, when we say it's dark, we mean it doesn't interact via any force except for gravity and perhaps the weak nuclear force. So any push or pull from dark matter on us would be undetectable. We call particles that don't interact with other matter except through gravity or the weak force, weakly interacting massive particles, or wimps. There are currently experiments underway looking for wimps, but by their nature they will be difficult to detect. Some of the experiments are looking for direct collisions with nuclei, while others are looking for the byproduct of wind decays or annihilations, if they even do decay or annihilate. This is an active area of research for physics, so hopefully in a few years we'll know more than we do now.