My tongue-in-cheek advice: "Everything" seems to be the word you are looking for!
There is however an old philosophical question behind this easy answer, i.e. if you want to include the things that do not exist as well, it gets complicated.
Another word might be: "Ontology". It usually refers to the philosophical study of concepts like being, existence, or reality as such. Example:
"Ontology is the discipline which…"
It is also used in another sense to speak of 'what there is', e.g.
"The ontology of a theory…"
refers to the objects (or relations or whatever) that the theory postulates or that one is committed to when believing a theory. In this second sense it might be a word which you can (ab)use in a sentence like
"I am not speaking only about the things we observe, but about the whole ontology [of the universe/cosmos]…"
Disclaimer: As a philosopher I would strongly discourage such an (ab)use of philosophical terms - it still sounds pretty impressive though ;)