The past is a learning tool that can be observed at some length, and a museum, like a library, is a place where knowledge and expertise can be accessed and given due reverence. Gaining spiritual knowledge in particular allows us to progress. A museum can represent a place where we store our memories and, therefore, can represent the subconscious it is that part of ourselves that we will usually only approach in an effort to understand who we are and where we came from. A museum in a dream denotes old-fashioned thoughts, concepts and ideas. We may need to consider such things but perhaps more objectively than subjectively. You might also like to consult the entry for Old.