Only about 4% of the universe is made of matter astronomers can actually see and measure. The rest, according to the leading model, splits between dark matter, roughly 27%, and dark energy, roughly 68%, neither of which has ever been directly observed. This Horizon episode puts cosmologists in front of the camera to explain how they infer something invisible from its effects: galaxies spinning too fast to hold together under their own gravity, and a universe whose expansion is accelerating rather than slowing down. The film also gives time to a minority position that treats the missing mass as a symptom rather than a substance, arguing Newton's and Einstein's gravity may simply behave differently at galactic scales and that a revised law of gravity could erase the need for dark matter altogether. No experiment has settled the dispute, and the episode lays out what each side would need to find to win the argument. It closes on an admission rare for a science program: the best current model describes 96% of the universe using ingredients nobody has caught in a lab.