Three of the most important words in science are ‘I don’t know’. Not knowing implies a universe of opportunities – the possibility of discovery and surprise. Our understanding of cosmology has advanced immeasurably over the last five hundred years of modern science, yet many fundamental mysteries of existence persist. How did our Universe begin, if it even had a beginning? How big is it? What’s at the bottom of a black hole? How did life on Earth arise? Are we alone? Is time travel possible? These mysteries define the scientific forefront, the threshold of the unknown. To explore that threshold is to gain a deeper understanding of just how far science has progressed. In this book, theoretical physicist and science writer Lawrence Krauss explores cosmology’s greatest known unknowns.