What is the Universe?

Here's a question: what do we talk about when we talk about the universe? Obviously, it's "all the things," but does that include the things that don't exist yet (aka the future)? And further, what about the tools we use to describe the universe (like math and physics) -- are those "part of the universe" or are they outside of it, in the same way that the rules of football aren't part of a given game of football? Interesting stuff, and worth two minutes to scratch your head a bit.

Sample quote: "If the universe is 'everything,' does that mean it includes the things that we don't know that we don't know exist? Or even the things we believe exist, but haven't yet seen or observed? For example, is the future a part of the universe?"

Now that you've gotten a taste of the fun physics here, read Calli Arcale's excellent article The Expanding Universe: How the Universe Got Bigger As We Measured It.