The physical universe is a most peculiar concept, not only different from the objects it contains but also from space itself. Cosmology - the science that seeks to explain it - has always differed from other sciences, drawing on philosophy and even theology. These influences remain, even in today's data-driven cosmological models. This accessible and far-reaching book traces the development of cosmological thought from antiquity to the twenty-first century, with particular focus on the foundational work of Einstein, Hubble and others in the early twentieth century. It explores how modern theories rest on a surprisingly fragile base and argues that many cosmic mysteries may remain permanently unsolved. Rich in historical and philosophical insight, this is a lucid guide to what we know - and perhaps cannot know - about the universe.