"Huh? What the...?"
As a result of the colliding energy between Gogeta's Super Kamehameha and Broly's own attack, the boundary of the dimension could not withstand the clash. Therefore, space and time "shattered" and Gogeta and Broly fell into this odd landscape. While Gogeta was surprised by this new environment, Broly was unfazed and continued his assault. It was here that Broly entered his Legendary Super Saiyan form while Gogeta became Super Saiyan Blue. They proceeded to fight through this odd landscape, and when they struck each other's fists hard enough, they fell out and ended up back on Earth.
The dimension appears to hardly have any terrain and gravity, as Gogeta and Broly were floating in the dimension most of the time during their battle. The only objects that seem to act as terrain are large asteroid-like objects that float around. Within the background are odd vibrant colors that circulate the landscape.