To the best of my knowledge, the top domains should be the domains from which your monitored services make calls to the services on the unmonitored hosts. They are considered the top because they would have the highest total time consumption out of the all the domains that are serviced by that host. I don't think there is a place where the entire list of domains are available as the concept would be to take care of the heaviest impacting domains.
David explained beautifully - just to elaborate a bit more, you could grab the information you are looking for from the Dynatrace API as well. I have not done this myself, but it may give you a more complete listing of all of the domains underneath the unmonitored hosts bucket.
More information if you're curious to try this can be found here: https://help.dynatrace.com/dynatrace-api/topology-...