History
Thought Space, as nicknamed by Mister Fantastic, was a dimension where a person's every thought manifested into reality. In an attempt to bond with The Thing, Mister Fantastic took him through the Forever Gate to Thought Space, where he willed into existence different scientific phenomena to study. Unaware of Thought Space's nature, Ben willed into existence an ever-increasing barrage of super villains for him to beat up since he was bored. Eventually, both Mister Fantastic and The Thing unwittingly thought of a future scenario where Ben murdered Reed, which had been previously shown to them by the Griever at the End of All Things and had lingered in their minds.
With the Forever Gate having been depleted of all its available uses in a month by Bently-23, who was unaware of Reed and Ben's trip, the duo found themselves stranded in this dimension. Mister Fantastic ultimately realized that with The Thing's help he could will into existence a construct of the Forever Gate, which they used to go back to their native dimension.[1]