At a first glance, our galaxy appears to be made of stars separated by vast and empty space. However, we now know that this space is filled with things that are more difficult to see – gas, photons and the elusive dark matter. We do not know what kind of particle dark matter is made of. In most theories it is heavier than the proton, present in space as a few particles per liter. However, there is another possibility – the axion, a particle originally imagined here at SLAC to explain a mysterious property of the neutron. If they exist, axions would be light and ghostly, streaming almost invisibly through the galaxy, and a single liter of space would contain a billion billion of them. This lecture describes the properties of the axion and explain how physicists are trying to observe this particle. The discovery of axions would not only revolutionize fundamental physics but would also open up a new, rapid and encrypted communication technology.