Sub-Space is an alternate dimension in the Gradius Universe. It is where the Bacterian Empire were born.
Nothing much is known about Sub-Space's history except what happened in 6840; Vic Viper reached the heart of the Bacterian Empire, which was located in Sub-Space. The organism that was the creator of the Bacterians (and also their center of power) was destroyed by Vic Viper, defeating the Bacterians. However, this did not mean the Bacterians were permanently destroyed; the pieces of the organism spread to other places which probably led to the rise of Doom, Life Force, and O.V.U.M..