The previous chapter was a whirlwind tour of the great advances made in our understanding of nature. These advances are associated with the icons of physics – names such as Albert Einstein, Marie Curie, Paul Dirac or Richard Feynman. For all their achievements though, we should not be beholden to great names. However much Einstein contributed, we now know far more, and understand it far more deeply, than Einstein did even at his intellectual peak. One important aspect of this understanding is that our understanding is incomplete. The theoretical jigsaw outlined in the previous chapter has several large and crucial pieces missing. We do not know for certain what these pieces look like, or where to find them, but we do know where they fit in.