Ever since humans looked up at the night sky, we have searched for clues that connect the celestial and terrestrial. However, it was not until the birth of modern science—seeking explanations with empirical data—that we came to understand our cosmic habitat better. And although we now know much about the universe we inhabit, there are many unsolved conundrums regarding the true nature of its key constituents: dark matter and dark energy. How do we know what we know about these enigmatic, invisible entities and what the pressing questions are at the frontier of cosmology?