Dark matter is the term astronomers use for the matter that they detect in the universe which cannot be seen. What I mean by this is that although when you look through a telescope nothing appears to be there, the way that the visible matter moves indicates that there must be more mass present. It is currently thought that about 10% of the matter in the universe is visible, the rest is something we don't understand.
Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.