Multiverse is a tab unlocked after completing your first Ultimate Multiverse Challenge. This tab is currently in development, with only two features as of May 2023.
The first thing you can start to do with your Multiverse is to make it stronger! Levelling up your Multiverse is similar to building monuments -- you assign clones and need a certain amount of prerequisite creations, in this case Universes, in order to increase the level. The speed of leveling is determined by your Creating Speed and the number of clones assigned.
The first Multiverse Level costs a total of 1e6 (1 million) Universes, with each one after that costing 5e6 * current Multiverse Level. So 5e6 more to get Level 2, 1e7 more (1.6e7 cumulative) to get to Level 3, and so on.
Each level will increase the four main god stats (Physical, Mystic, Battle, Creating) by 100%, making it very useful when climbing P.Baals.
The time needed and the Universe cost to level the Multiverse will increase each level. Completing UMC can reduce the time needed to level, by 5% each challenge completion for a total of 100%, or twice as fast.
Completing a DMVC unlocks the Boost, which acts as a multiplier to your Multiverse Level's stat bonus. As with the ML, it begins at level 0, and can be increased 1 level at a time by assigning shadow clone workers. However, unlike the ML, this one doesn't cost universes. It costs divinity per second, which is subtracted directly from the Divinity Generator's output.
The div/s required to build a Boost level is proportional to the number of shadow clones assigned, and also depends on your DMVC score. If the DivGen isn't producing enough, then the Boost will take all of the available div/s, and any extra clones will effectively do nothing. The CAP button will assign as many clones as the DivGen's output can support (or your current number of available clones, if that's smaller).
The full stat bonus provided by Levelled and Boosted Multiverse is:
(100 + Multiverse Level * 100) * (1 + Boost Level)%