Okinrus,
I thought the universe only defined this particular world.
You didn’t really think that did you?
That was the thinking of the bible authors and why most of what they wrote is of little value to us now.
I've also heard many theories of multiple universes.
These tend to be miss-uses of the term since they depend on a limited popular concept of what is meant by universe.
Current theory from physics describes multiple universes as bubbles within a single universe. I.e. the material and energy that forms from a big bang would be one such bubble, which most people think of as an entire universe. The theory includes the possibility of a potentially infinite number of bubbles co-existing and at different stages of evolution, all within a single universe.
Science fiction also includes the concept of parallel universes where the current universe is repeated and phase-shifted and/or time-shifted through an infinite number of minute variations. There are many versions of these themes including the concept of effective time-travel where the universes are all identical but time-shifted from each other – so to travel through time one simply travels horizontally through the parallel universes.
If there is a God then the universe must be created by him.
God with an upper case G is the name given to the god of Christianity, although Islam and Judaism use the same name. Such a god is defined as a creator of the universe, so I agree if such a god existed then it must have created the universe.
However, if you are using the sentence as an attempt at logic then it is invalid and/or ambiguous.
If the universe is infinite, i.e. has no beginning, then a creator god could never exist. This does not rule out the existence of a god but it would have to be defined differently.
Also the universe is expanding into empty space, but is the empty space part of the universe?
That is a somewhat simplistic view of the universe. If empty space can be said to exist then it would be part of the universe. If your view of the universe is a single big bang bubble then it would be more accurate to say that the universe is expanding. There is no need to say it is expanding into something. However, I understand it is intuitive to think of it that way.
We could also consider the universe as infinite in size, in which case there is nowhere for it to expand, but I’m sure you are thinking only of the single big bang perspective. But this would solve the expansion into empty space problem. However, the theory of multiple bubbles also considers the possibility that each bubble might be formed with slightly different rules of physics. It is not clear what would happen if two such dissimilar bubbles collided. Current thinking is that the distance between them would be so great that they would expand and dissipate before they ever met.