Hollow was created thousands of years ago by aliens, known only as the Progenitors or Progs. No-one knows why they made a hollow, inside out world but they seemed to have lived in it for a while then left.
Only a few traces of the Progs remain and very little is known about them. Even their reason for building Hollow in the first place is a mystery but it is widely believed it was built in the space between universes as a stepping stone to wherever it was the Progs went.
While they still inhabited Hollow the Progs began filling it with people, animals and plants from all over the universe.
The ethics of picking up people from their home world without their permission and dropping them in a new artificial world is somewhat questionable. It tells us that the Progs were collectors who apparently thought it would be nice to have someone to look down on and weren’t too bothered about how it would traumatise the collectees.
As far as anyone knows, none of the intelligent species the Progs chose to bring to Hollow were beyond an early industrialisation stage when they were selected. In the known regions of Hollow there are no advanced technical civilisations.
As well as the people, the Progs also imported flora and fauna from many worlds. To make it possible for all the different living things to exist together, the Progs developed nano bots – tiny machines the size of bacteria – that protect the various living macro organisms from the legions of alien-to-each-other micro organisms swilling about. The nano bots aren’t perfect though and diseases still occur. This provided a perfect cover story for a certain living skeleton who preferred to be swathed in scarves, bandages and clothes to conceal their true nature while telling everyone the covering was due to a disfiguring skin disease.
Certain species the Progs collected came from worlds with unusual conditions, such as atmospheres toxic to others or gravities that were much greater or lesser than the standard in Hollow. For these special cases, the Progs created zones that replicate the conditions on these species’ home worlds. The zones are surrounded by force fields that contain these special conditions and prevent those dwelling within the zones from leaving.
Interestingly, the force fields do not prevent other species from entering zones – or leaving if they are still alive.
Zones were also created for species that are excessively dangerous to others. An example is the Mattins Zone which is populated by beings who enjoy hunting other intelligent species and killing them as bloodily as possible.