"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.
Having been pushed to the breaking point after continuing to be at a disadvantage, Broly finally entered his Legendary Super Saiyan form and managed to overpower Gogeta. But in response to this, Gogeta changed into Super Saiyan Blue which turned the battle back into his favor. They proceeded to fight through this odd landscape until they struck each other's fists hard enough that they fell out and ended up back on Earth, where they continued the rest of their battle.
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.
Video Game Appearances
In Dragon Ball Z: Dokkan Battle, the dimension of strange swirling lights is referenced in base Gogeta's card art and his Super Saiyan God SS state Super Attack, the latter in which he performs the same Stardust Fall combo as he did against Broly.