Every scientific fact was born as an opinion about the unknown - a hypothesis. Opinion gradually becomes fact as evidence piles up to support a theory. But what if there are two theories, each of which has produced a myriad of things that correspond perfectly to the phenomena but can't be combined into one? One theory replaced the mystery of gravity with a precise model of space and time. The other theory replaced the mystery of matter with a description of quantum particles. As we understand our universe, we keep each in its own space and time for very large things, particles for the very small ones. However, 13.8 billion years ago, those two incompatible domains belonged to a single realm. Who in the current or future generations of physicists will crack this seemingly impossible puzzle? This, contends the author, is not just a big question, but the biggest question in physics in our century. Combining Ickes' first-hand knowledge with a robust argument and intellectual playfulness, this fascinating book succeeds in making a notoriously difficult subject accessible to all readers interested in a better grasp of our universe.