Monism is false, because there are surely different ontological categories like events, space-time, relations, minds( if there are any), abstract objects( if there are any), and concrete objects etc. The universe cannot be an individual from being the mereological sum of all the objects in all the categories, because we know that mereological sum have no unique decomposable identity, but there is surely a uniquely decomposable identity here with respect to these categories. The universe cannot be a set, for a set in a abstract object, and there are objects that are not abstract. The universe cannot be a class for similar reasons. Thus, the universe cannot be a individual, so there has to be two are more categories. One way to identity "the universe" is set it to refer to all objects of every categories( if those categories exist). Even so, we want to be more specific, but yet not commit to any underlying ontology. There could be disjointed space-time etc. There are two, but not mutually exclusive ways to thing about it. Either there is some type of principle that lists all objects of a certain kind are in the universe, or that there could be as list of complete different things like: the number 7, my keys, rocks, social security numbers...etc...

In conclusion, the universe is not an individual, but it can still have extension to all objects of all categories( for all those categories that actually exist). If so, then the universe can either( inclusive) have some organization principle that dictate what can exist, or that we have a list of things that are not in any way related.