Why ask what empty space really is? It is the nature of space itself that seems to be at the heart of the mysteries in physics and cosmology today. Einstein also told us that space is a something. He showed it could be stretched and warped when he published General Relativity in 1915. But over the last one hundred years, the nature of empty space has continued to become more mysterious and confusing. We've learned a lot about the Universe in the last one hundred years. We've even learned a lot about the nature of empty space. But we still have a lot more to learn! That is the subject of this book.