Cosmology concerns itself with such ideas as Form, Order, and Structure, as these ideas apply to the Universe at large. In effect, we use what we have learned from Ontology to help us understand the kinds of structures—even the basic structures of Space and Time—that actually do exist. And of course, whatever we say or think here must not contradict what we have decided upon in Ontology. All the same, it seems obvious enough that determining what exists physically is not without its difficulties, if only because of the derivative or secondary nature of the experience we obtain through the windows of our senses.
Perhaps the least problematic kind of entity is the material entity, and in fact some philosophers consider our experience of the physical world to be primary. As anyone with a robust sense of the onticity that, as science seems to imply, exists independently of our thoughts will agree, material things constitute fundamental kinds of entities, and our understanding of all other kinds of entities—things like Minds and Numbers, Societies and Relationships—derives from our understanding of these fundamental material entities. This, then, is a good place to begin; at least, it's as good as any. And among material entities, it seems quite natural to say that the most important material entity each of us knows is our own body. We shall begin our Cosmology, then, with the existence of our own bodies.