Dark matter is a cosmic enigma. We can't see it or touch it -- so what is it? We asked MIT physicist Max Tegmark about the nature of this strange substance and why it remains so mysterious. Produced by David Levin. Interview by Julia Cort. Major funding for NOVA is provided by The DOW Chemical Company, the Corporation for Public Broadcasting, and public television viewers. Major funding for NOVA scienceNOW is provided by the National Science Foundation and the Howard Hughes Medical Institute. Additional funding is provided by the Alfred P. Sloan Foundation. For more fun science stories, visit http://www.pbs.org/nova/sciencenow Less