The mature different spaces mentioned by the sea breeze whale actually pointed at the countless different dimensional planes in every major region of the Origin World.
They were similar to the Fifth Dimension, Sixth Dimension, and Seventh Dimension on his Earth. They contained all sorts of fantastic oddities which far surpassed anyone's imagination, but it wasn't like Earth where they had only a few stable different planes.
As for why the whale insisted on using the word 'mature'?
This was the most important point. Only those dimensional spaces which had stabilized over countless years, having not collapsed for several thousand years could be considered as mature different dimensions.
People weren't willing to enter dimensions which hadn't stabilized. Well, besides the few extremely formidable lunatics who didn't care much about their life.
