The Amazing World of Gumball is a very surreal show. There are characters animated in multiple art styles and backgrounds taken directly from the real world, creating an almost technicolor feel. But beneath its cheery exterior is something completely different, almost malevolent if one were to call it that due to the mysterious force trying to move against the shows world.
Elmore (and possibly the whole universe) contains a pocket dimension known as The Void which erases people and objects it deems mistakes. These mistakes include embarrassing fads, historical disasters, and even characters from the show. Usually, these purges happen with no consequences because everyone's memories of the "mistakes" are erased. The mistakes possessions are removed as well, which are easily noticed due to the obvious folds where their houses once were. These memories usually don't come back unless one is wearing a tinfoil hat, preventing the world from erasing their memories.
This enigma has been present since the middle of season two, in which Gumball and his family begin to grasp the true meaning of the world. An example of this is in The Finale, an episode about The Wattersons discovering that their past misdemeanors have severe consequences in the present which is made apparent by the mob of townspeople demanding retribution for the various acts done against them like Mr. Small suing the family for giving him giving him claustrophobia the time Gumball and Darwin got him stuck in the filing cabinet.
Another example is in The Kids, when Gumball and Darwin have their voices changed, the screen turns to static for a second as if something is altering reality for the betterment of the show. Similar hints are dropped throughout the season, but the biggest points so far have been The Void and The Nobody.
In the former, Gumball and Darwin are on the hunt for a girl named Molly, a sauropod girl who was often seen in the corridors of Elmore Junior High during Season 1. It turns out that Molly was removed from the world itself and put into The Void due to her personality being bland and uninteresting to fans. Gumball, Darwin, and their guidance counselor Mr. Small manage to open up one of the folds in the world itself and that creates an opening to the unexplainable.
Once inside, Gumball and Darwin find Molly in her treehouse while Mr. Small finds his van Janice, which was possibly removed from the real world due to it's eccentric appearance. Everyone manages to escape the void before the portal folds in on itself and they celebrate Molly's rescue before the tinfoil hats they were wearing got sucked into the void.
In a later episode called The Nobody, Gumball and Darwin learn that Rob ( the blue cyclops kid) managed to escape the void and has been living in The Wattersons basement for quite a while. He makes it his duty to become their nemesis and swears to destroy everything they care about and take away everyone they ever loved.
Despite this, it turns out he is a mere annoyance in the second episode of season four, The Nemesis, where he keeps setting up traps for the duo but to no avail. They manage to convince him to take his nemesis duties seriously and this turns him into Dr. Wrecker, who is even more pitiful. He tries destroying the Elmore Dam, but it turns out it was the Auto Repair Center and Rob just made a vending machine malfunction. While he may not appear threatening, he may have something to do with the second half of season four, which I will now discuss:
Subtle hints will be dropped through the later portion of the season and at some point the shows world will be engulfed by The Void and all its mistakes will run rampant through Elmore, being led by an arrogant Dr. Wrecker, The Wattersons will become trapped in the void (That was foreshadowed in The Oracle, the episode where Banana Joe's mom paints a prophetic painting of Gumball's naked run through the mall), they will be replaced with cheap knockoffs, and no one will remember them ever existing except for a select few.
This is made even more plausible when a list of Gumball episodes accidentally leaked on Cartoon Network's website and among them was an episode named The Disaster, which is apparently the first two part episode in the series. This is just my two cents and isn't intended to be taken seriously due to the unpredictable nature of the show itself.
The Amazing World of Gumball airs weekdays on Cartoon Network