In tonight's episode, we take a look at two mysteries that keep sparking debates in their respective scientific communities: - The Cambrian Explosion: at the beginning of this period of our planet's history, the number of life forms and their complexity increased dramatically - both in terms of body plans and behaviours, with the emergence of ecological complexity, like predator-prey relationships. But why did it happen? - Dark Energy: like Dark Matter, Dark Energy is a hypothesis, a way of reconciling observations with the laws of physics, especially gravitation. The Dark Energy hypothesis arose with the observation that the expansion of the universe was accelerating instead of remaining constant or slowing down, and this theoretically requires energy. But what could this mysterious energy be: an unknown fundamental force, fields of particles, an energy of the vacuum, something else...?