The current Standard Model of Cosmology has successfully explained many phenomena, but it predicts that the majority of the Universe consists of dark matter and dark energy, whose properties are poorly understood. Because huge volumes collapse into galaxy clusters, the largest known gravitationally bound structures, they are an ideal laboratory to study the Dark Universe. In fact, the ratio of baryonic matter to total matter in a massive cluster, fgas, can be considered representative of the matter content of the Universe as a whole. Measurements of fgas from the heaviest, dynamically relaxed galaxy clusters place powerful constraints on cosmological parameters as well as the dark energy equation of state. I will discuss constraints derived from fgas measurements using a multi-wavelength set of X-ray and optical data and provide outlook on the future of this measurement in the age of precision cosmology.