I, too, look at the universe as being all encompassing. That said, how the universe came to exist is (and will most likely be) the unanswered question in the end. Depending upon one's ideologies, one may believe in the Big Bang belief, while others may hold to their own religious ideologies.
I would second post #3's comment-- the universe is the sum of everything. The universe is the really big mixing bowl that holds it all together: all the stars, galaxies, worm holes, space dust--everything.
It sounds silly, but I just think of the universe as being "everything." If it exists, it's part of the universe. Of course, people who are well versed in modern physics might tell us that there are multiple universes. That's too much for my brain to handle, but who knows?
What question could be bigger than this? It is one of the most timeless ponderings of the human race--what is this place we're in and how did it come to be? Since the title of this discussion is "The Universe Definition," let's start with just that--the word itself.
According to my etymological dictionary, "universe" comes from the Latin "universus." This word came from "unus," meaning One and "versus," which is the past participle of the Latin word for Turn. So, the word "universe" literally means "turned into one." I find that extremely interesting. Here's why...
As most of us know, the scientific community generally is in consensus that the Big Bang or something like it must have been the catalyst for the creation of the universe as we know it. They have lots of evidence for this, one of the main points being that the universe continues to expand outward, and the laws of physics require an original, powerful force (the bang!) to have been implemented in order for that to happen. There are also people with strong religious beliefs about alternative methods of creation, namely that of an intelligent deity who was able to call the universe into being. Now, Latin is the language of the Roman Catholic empire, but also that of scientific classification. And the original Latin definition of the word "universe"--turned into one--really fits either and/or both of these ideas. Out of all existing or potential matter, energy, space, and time, appeared the single endless, wondrous expanse of this realm that we inhabit. Personally, I believe in God and Science, and turn to both to understand the unfathomable.