The space between the stars is not empty at all, in fact, there is on average one particle per cubic centimeter in the so-called interstellar medium. This is still significantly more tenuous than even the best vacuum chamber on Earth, but since the interstellar medium is so vast there is enough gas in it to be visible for some telescopes. One of the most reliable ways to probe the interstellar medium is the 21-cm radio line of atomic hydrogen, by far the most abundant gas in space.
In my research I want to compare observations of this 21-cm line with cosmological simulations – computer models which try to emulate a synthetic universe. This work should help improving the way how cosmological simulations model the interstellar medium, and might also prove interesting for some observational aspects of 21-cm radio astronomy.