We can't see dark matter, and some skeptics doubt its existence, but many scientists think it makes up 20-some percent of our universe. Astronomer Doug Clowe explains how the Bullet Cluster, a group of galaxies billions of light years away, may shed some light on this mysterious stuff.

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Video podcast produced by Melissa Salpietra and Susan K. Lewis. Edited and narrated by Melissa Salpietra. Original footage and interviews produced by Julia Cort. NOVA is produced by WGBH in Boston. Funding for NOVA scienceNOW is provided by Pfizer, the National Science Foundation, the Howard Hughes Medical Institute, the Alfred P. Sloan Foundation, the Corporation for Public Broadcasting, and public television viewers.

This material is based upon work supported by the National Science Foundation under Grant No. 0407101. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.