“What is everywhere, not made of atoms, and can’t be seen?” Dark matter, says renowned astrophysicist David Spergel—but not everyone in the cosmological community is in agreement with him. This program presents the views of Spergel and other key figures in the debate, including Princeton University’s P. James Peebles and Jeremiah Ostriker; Timothy Sumner, of Imperial College London; astrophysicist Mordechai Milgrom; and Saul Perlmutter, a member of Lawrence Berkeley National Laboratory’s physics division. Experiments in Europe’s deepest mine looking for the elusive neutralino, the concept of variable gravity, and what may well become the new standard model of how the universe works are all scrutinized. Original BBCW broadcast title: Most of Our Universe Is Missing. (50 minutes)