Ever since we first recognized that the Universe was expanding, one question has dominated the minds of those who study it: how will it all end? Today, we recognize that our Universe began from a much smaller, denser, hotter, and more uniform state: a state that we identify with the hot Big Bang. Over time, that Universe has expanded, cooled, and evolved, but also gravitated: clumping and clustering to form an intricate cosmic web of structure across millions and even billions of light-years. To...