Useless Fade...
(Lore Bombs ahead)
(Explaining a few results of various types of research done by humanity's greatest scientists on the phenomenon called 'Dungeon gate opening'. It is recommended to not skip this explanation to better understand the basic workings of this world.)
The Space-time distortion theory:
Several researchers have found that the inside space of a 'Dungeon gate' is most of the time an imitation of a particular area in the universe.
It is speculated that the area inside a dungeon gate is an imitation of what lies outside the actual world. To prove this theory true, the scientists compared many real-world and dungeon-world environments, and it was found that this theory has some weight in it.