Dummy topics can be seen in insights and reports that show data on a topic level. Dummy topics are automatically created by Universal Analytics, and are used to group free-standing items of content that do not currently exist inside a topic. Dummy topics only exist in Universal Analytics and do not actually exist in Fuse.
Ideally, dummy topics should be filtered out of insights to give an accurate view at topic level. In some cases it may be useful to include dummy topics. For example, you might want to compare content engagement in dummy topics vs real topics. This allows you to see if engagement increases or decreases when content is included in topics or when simply uploaded to Fuse individually.