In 1972, Faber joined the faculty of the Lick Observatory at University of California, Santa Cruz, becoming the first woman on staff. In 1976, Faber observed the relationship between the brightness and spectra of galaxies and the orbital speeds and motions of the stars within them. The law that resulted would become known as the Faber-Jackson relation, after herself and the co-author, graduate student Robert Jackson. Three years later, she and collaborator John S. Gallagher published a seminal paper collecting all of the evidence for the existence of dark matter that had been published at that point. In 1983, she published original research showing that dark matter was not composed of fast-moving neutrinos ("hot dark matter") and that instead, it was likely composed of slow-moving particles yet to be discovered ("cold dark matter").