Astronomer Lindsay House returned to the podcast to update us on her citizen science project called Dark Energy Explorers. Around the globe, 20,000 explorers are using data from the world’s third largest telescope to observe patterns in the movement of galaxies between 9-11 billion years ago, to understand how the expansionary force of dark energy operates. As Lindsay explains, dark energy, which comprises 70% of the universe’s mass, is undetectable to our instruments but exerts an expansionary force by adding space between physical objects. Since 1998, we’ve known that over time the universe is expanding at an increasing rate that’s long thought to be constant, but new evidence is pointing to a reduction in the speed of expansion. What function is dark energy exercising in time, if its force can both increase and decrease universal expansion over time. Would its variability lead to a revision of the “lambda-CDM” model of cosmology that’s based on a constant expansion rate? The observations of the Dark Energy Explorers community from 159 countries, sharing knowledge translated in 9 languages, are contributing to expanding knowledge of the still-mysterious nature of dark energy. The Dark Energy Explorers are, as Lindsay says, the humans “in the loop” as the capabilities of AI and machine learning help to make vast numbers of groundbreaking connections with the astronomical data. Hear Lindsay tell the story of Dark Energy Explorers, and how scientists are coming to understand more about the fascinating story of the invisible force of expansion.