Current cosmological observations suggest that we live in a universe dominated by dark matter and dark energy while their nature remains elusive. Compelling evidence for the existence of dark matter derives from the gravitational influence exerted on the motions of stars and galaxies as well as the bending of light. Meanwhile, finding the dark matter particle has been challenging due to its inertness. Despite this, we have a successful dark matter-based theory of structure formation in the universe. Although the theory has been validated from many independent lines of compelling evidence, there are a few gaps—a few observations that challenge the paradigm. At this juncture a comprehensive exploration of all possibilities is warranted, including those that radically question the current picture to help frame and refine alternatives. Alternatives to the dark matter model include modifications to Newtonian gravity; generalizations of Einstein’s theory of general relativity; and extensions to the standard model of particle physics that open up an entirely new window—the dark sector—for the putative dark matter particle. Seminar participants consider the status of the current cold dark matter picture; devising and assessing alternative models, particles, and theories; and debating whether and where a radical rethinking is likely to take us and how we might be able to test and verify such alternatives.