This study examines the practical work of a pair of students and an instructor using probeware in a mechanics lab. The aim of the study is to describe and discuss a type of interactional sequence that we refer to as "dark matter", the ordinary backdrop to the extraordinary sequences that are easily recognizable as clear-cut instances of learning. Although this work is downplayed in the research literature, describing it is critical to properly understanding lab work as an educational practice. With a focus on the negotiation of disciplined perception, we analyze a number of episodes wherein a pair of students and an instructor struggle with the construction and interpretation of a graph depicting a linear relationship between force and acceleration. We demonstrate an intimate interplay between how the students display their problems and understandings and how the instructor tries to make the subject matter content visible and thus learnable. The analyzed episodes are illuminating with regard to the analytical notion of disciplined perception as applied to graph interpretation; the cognitive and practical competencies involved in producing, recognizing, and understanding graphs in mechanics; and the interactive work by which these competencies are made into objects of learning and instruction. (Contains 6 figures and 8 footnotes.)