A lecture delivered by UCI Professor James Bullock on February 11, 2009. James Bullock, Associate Professor of Physics and Astronomy at UC Irvine, is part of a team of scientists who believe they have discovered the minimum mass for galaxies in the universe -- 10 million times the mass of the sun. This mass could be the smallest known "building block" of the mysterious, invisible substance called dark matter. Stars that form within these building blocks clump together and turn into galaxies. Dark matter governs the growth of structure in the universe. Without it, galaxies like our own Milky Way would not exist. Dark matter's gravity attracts normal matter and causes galaxies to form small galaxies and to merge to create larger galaxies.