Retroduction, Multiverse Hypotheses and Their Testability

The actual existence of collections of universes -- multiverses -- is strongly suggested by leading approaches to quantum cosmology, and has been proposed earlier as an attractive way to explain the apparent fine-tuned character of our universe. But, how can such hypotheses be tested? After briefly discussing the key distinction between possible and really existing multiverses, and the importance of an adequate generating process, we focus on elaborating how multiverse hypotheses can be retroductively tested, even though they will probably never be directly observed.In this approach, scientific acceptance of multiverses would rely on the long-term success and fertility of quantum cosmological theories including them as essential elements or as inevitable consequences.