The Universal Gateway was the one universe that connected to every single universe.
The Universal Gateway was the first of all universes, although it was just an indefinite amount of space at that time. One of the first notable uses would be in the Jimmy Timmy Power Hour at that time.
The Gateway evolved to become a purple hammerspace, which then it was used in Battle Calls.
In 2013, just as the Gateway became a complete universe, the Multiverse Collision began. The Gateway was used as an arena for the five collisions.