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. NOVA is produced by WGBH in Boston.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.This material is based upon work supported by the National Science Foundation under Grant No. 0229297. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.For more fun science stories, visit our Web site at http://www.pbs.org/nova/sciencenow