More at http://philosophytalk.org/shows/dark-matter.

All the matter we have ever observed accounts for less than 5% of the universe. The rest? Dark energy and dark matter: mysterious entities that we only know about from their interactions with other matter. We infer their existence to satisfy our laws—but are we justified in making conclusions about what we cannot directly measure? How far can we trust our scientific laws? Where do we cross the line from theoretical science to metaphysics, and can the two overlap? John and Ken see the light with Priya Natarajan from Yale University, author of "Mapping the Heavens: The Radical Scientific Ideas That Reveal the Cosmos."