acronym for Mysterious Unknown Stuff that “must” exist to keep a theory from collapsing: e.g., dark matter, dark energy. The current philosophy of science in vogue, called “scientific realism,” reserves the right to consider “unobservable reality” if it serves the “best” theories – but what is considered best is subjective, varying according to the whims of the consensus or paradigm (if one is a Kuhnian). Appeals to “occult forces” like phlogiston, epicycles or caloric have plagued the history of science. At some point, the proposed placeholder must be confirmed by evidence, or it becomes indistinguishable from confabulating. Source: 2/28/08.