The Solar Wind
Space isn't empty, especially not in the Solar System, it is filled with a stream of particles given off by the sun called the Solar Wind. Well, when I say "not empty", I mean "almost not empty" . If you take an imaginary cube of space, one centimeter on all sides, it will be filled with about 10 particles of solar wind. On the Earth, if you were to fill it with the atmosphere you are breathing now, it would be filled with 20,000,000,000,000,000,000 particles of air. The solar wind is made up of mostly protons and electrons (which are tiny bits that make up atoms), and travels very fast.
A bullet from a high powered hunting rifle travels at about 1 kilometer per second. The Space Shuttle orbits the Earth at about 8 kilometers per second. The Solar Wind travels anywhere between 200 to 700 kilometers per second!
So how do you actually measure this near vacuum? That's what my Ph.D project is all about. I am designing instruments called "Top Hat Hemispherical Analysers". They are scientific experiments that measure not only how many solar wind electrons there are, but also how fast they are moving. From this, scientists can find out all sorts of important information and learn about space plasma. My group has worked on Plasma Analysers from five un-manned robotic missions that are in space right now.