This talk was given at TEDxOporto 2016 to a large, general public audience. It attempts to address the question, posed by a young student: "How do we measure what we can't see?" The talk addresses this in three parts: What we currently know about our universe How we came about to understand this, through exploration and extrapolation (experiment and theory) How we can continue this process to make new discoveries, such as the composition of Dark Matter Note that Powerpoint needs to be used, in order to take advantage of the animations, especially the 100 slides on the composition of our universe.