What is dark matter?
Dark matter is one of the biggest mysteries in what we understand about the universe. The great progress in the last ten years has meant that we know how much stuff there is. We know that because we can see how the universe is expanding and that expansion changes with time because of the gravity of all the stuff that is within the universe. So if you have lots of stuff you have strong gravity and the universe gets pulled together, if you have not much stuff then the universe expands without really being affected by the matter within it. Now, we can count up all the stuff we can see, we can weigh galaxies by looking at their stars, adding up the gas, the stars, all the matter that we know about and its not enough to match the measurements of the universe's expansion. In fact, all normal matter, everything that you and I are made of, everything the earth and solar system are made of adds up to a sixth of the total matter in the universe. So the question, what's the rest of it? Well we don't know so we call it dark matter. We see its effects by its gravity but that's it, it doesn't seem to interact with the rest of the universe and there is some around you right now, wherever you are sitting. The theory tells us that there is something like a hundred thousand dark matter particles passing through your body every second, but they don't interact, they have gravity but they don't interact with normal matter in any other way.
