The Black Garden

Anyone got any theories on the Black Garden? Could it be Mercury? Or could it be a whole other pocket universe, that the Vex created to breed other Vex, without any interruption or time taken to do this ( since the Garden is not within time, the Vex could create a whole army, and then put it in the present time ) And what is that mysterious Rock/Uluru like structure at the back? I call the 'Rock of Solace' ( please use this name when speaking about it just so not to confuse anyone )