In 2008, the phrase "big data" shifted in meaning. It turned from referring to a problem and an opportunity for organizations with very large data sets to being the talisman for an emerging economic and cultural order that is both celebrated and feared for its deep and pervasive effects on the human condition. Economically, the phrase now denotes a data-mediated form of commerce exemplified by Google. Culturally, the phrase stands for a new form of knowledge and knowledge production. In this essay, we explore the connection between these two implicit meanings, considered as dimensions of a real social and scientific transformation with observable properties. We develop three central concepts: the datasphere, thick mediation, and representational opacity. These concepts provide a theoretical framework for making sense of how the economic and cultural dimensions interact to produce a set of effects, problems, and opportunities, not all of which have been addressed by big data's critics and advocates.