When do people say that an event that didn’t happen was a cause? We extend the counterfactual simulation model (CSM) of causal judgment and test it in a series of three experiments that look at people’s causal judgments about omissions in dynamic physical interactions. The problem of omissive causation highlights a series of sub-problems that need to be addressed in order to given an adequate causal explanation of why something happened – what are the relevant variables, what are their possible values, how are putative causal relationships evaluated, and how is the causal responsibility for an outcome attributed to multiple causes? The CSM predicts that people make causal judgments about omissions by mentally simulating what would have happened in relevant counterfactual situations. People use their intuitive understanding of physics to run these mental simulations. While prior work has argued that normative expectations affect judgments of omissive causation, we suggest a concrete mechanism of how this happens – expectations affect what counterfactuals people consider, and the more certain people are that the counterfactual outcome would have been different from what actually happened, the more causal they judge the omission to be. Our experiments show that both the structure of the physical situation as well as expectations about what will happen affect people’s judgments.