Dark energy makes up nearly 70% of the universe, yet nobody can say what it actually is. This film from Spark lays out how astronomers stumbled onto the problem by watching distant exploding stars, supernovae that revealed the universe isn't just expanding but accelerating, a discovery that upended a century of assumptions built on Einstein's general relativity. It traces Einstein's own "cosmological constant," a fudge factor he once called his biggest blunder, back into fashion as the leading explanation, and sits with the embarrassment of quantum vacuum calculations that miss the observed value by a factor of 10^120. Physicists and cosmologists explain how space telescopes and atomic vacuum traps are being built to pin the phenomenon down, and what it would mean if the answer requires rewriting gravity itself rather than just adding a new ingredient to it. The film closes on the open question of whether current physics is missing something fundamental, or waiting on an Einstein-level rethink.