An alternate universe, or alternate dimension, was an alternate version of reality in which a past event had led to different – often drastically different – consequences. In the BIONICLE universe, new alternate universes were continually being formed, as every decision or event resulted in the creation of an alternate universe where the opposite outcome took place. There was no designated main universe; countless alternate realities existed, and the concept of a main reality was simply a matter of perspective.
For a list of significant alternate universes, see List of Alternate Universes.
When Tobduk came to the fortres of Destral to kill Makuta Tridax and noticed the copies of Takanuva kept in cases there, he remarked, "Someone's been tampering with things best left alone. I've heard enough Turaga tales to know what that leads to."
When Mazeka and Vultraz went through a portal to an alternate universe, they experienced a sickening moment of darkness and disorientation before emerging in the new universe.
In the Melding alternate universe, the Great Beings were aware that they had counterparts elsewhere in the vast, uncounted realities that existed. Because of this, Mazeka and Vultraz's appearance was fascinating, but not a surprise to them. They thought that it was only a matter of time before one of their creations pierced the dimensional walls. In return for keeping Vultraz in their universe, they allowed Mazeka to take back a being from their universe back to his own, to maintain balance between the two realities.
An alternate dimension was formed whenever a decision was made in the main universe.
Any decision resulted in the creation of a universe where the opposite decision was made.