This 20 Answer Cosmos: A Spacetime Odyssey Episode 13 worksheet is designed to accompany Episode 13, Unafraid of the Dark, of the documentary television series Cosmos: A Spacetime Odyssey hosted by Neil deGrasse Tyson and follows up Cosmos: A Personal Voyage presented by Carl Sagan. Tyson begins the episode by noting how the destruction of the Library of Alexandria lost much of humanity's knowledge to that point. He then contrasts on the strive for humanity to continue to discover new facts about the universe and the need to not close off further discovery. Tyson then proceeds to describe, through a roundabout way, the discovery of cosmic rays by Victor Hess through high-altitude balloon trips, that radiation increased the farther one was from the surface. Fritz Zwicky, in studying supernovae, postulated that these cosmic rays originated from these events instead of electromagnetic radiation. Zwicky would continue to study supernovae, and by looking at standard candles that they emitted, estimated the movement of the galaxies in the universe. His calculations suggested that there must be more mass in the universe than those apparent in the observable galaxies, and called this dark matter. Initially forgotten, Zwicky's theory was confirmed by the work of Vera Rubin, who observed that the rotation of stars at the edges of observable galaxies did not follow expected rotational behavior without considering dark matter. This further led to the discovery of dark energy by Edwin Hubble to account for the known rate of expansion of the universe beyond the visible and dark matter mass. Tyson then describes the interstellar travel, using the two Voyager probes. Besides the abilities to identify several features on the planets of the solar system, Voyager I was able to recently demonstrate the existence of the Sun's variable heliosphere which help buffer the Solar System from interstellar winds. Tyson describes Carl Sagan's role in the Voyager program, including creating the Voyager Golden Record to encapsulate humanity and Earth's position in the universe, and convincing the program directors to have Voyager I to take a picture of Earth from beyond the orbit of Neptune, creating the image of the Pale Blue Dot. Tyson concludes the series by emphasizing Sagan's message on the human condition in the vastness of the cosmos, and to encourage viewers to continue to explore and discover what else the universe has to offer. Applies to science, astronomy, physics, history, chemistry, physical science, and Earth science.