Everything you can see or touch makes up only about 5 percent of the universe. This episode digs into dark energy, the roughly 68 percent that is accelerating cosmic expansion. The hosts begin with Einstein's cosmological constant, Hubble's 1929 discovery of expansion, and the 1998 supernova surveys that found Type Ia standard candles dimmer and farther than gravity allowed, a result that earned the 2011 Nobel Prize. They explain the Chandrasekhar limit that makes those explosions reliable rulers and why dark energy's constant density lets it dominate as space grows.

Independent evidence follows from the cosmic microwave background, a flat universe requiring a missing 68 percent, and baryon acoustic oscillations frozen into galaxy distributions. The discussion then covers the leading explanations and their problems: vacuum energy that overs ...