Marioplex is a large number named by YouTube user MatPat in a video called "Game Theory: Super Mario Maker, BIGGER than the UNIVERSE!". It is equal to 1012,431, and therefore (after great googolplex) the second largest googologism which is a power of 10 with an exponent coprime to 6.
MatPat invented this number in August 6, 2017, when trying to calculate the number of possible levels in the game Super Mario Maker.
This number was calculated by repeated use of the fundamental principle of counting, and represents, very roughly, the fraction of the number of possible levels in the aforementioned game that are both fun and completable.
As the exact number of levels with said properties is impossible to calculate, a Fermi estimate was used to define the number.
In fact, many googols of copies of the universe would be needed to have even a small fraction of a Marioplex of Planck volumes, the smallest possible amount of volume that is thought to exist.