What is universe ? Wikipedia says "The Universe comprises everything we perceive to exist physically, the entirety of space and time, and all forms of matter and energy"
There is an interesting word in this definition, "perceive" that is defined as "perception is the process of attaining awareness or understanding of sensory information". Perception is a subjective term. It is not the same way that I perceive and you perceive. Then we all should have an our own universe.
Let's define my universe as "Everything I know" where your universe is "Everything you know". I know that there exist objects (matter or energy) beyond the farthest object that I know and there is a time before the earliest event I know. Hence universe (I use the word universe for my universe) is infinite in size (the space time). (Remind the definition for plus infinity in mathematics that is "it is greater than any real number"). But the distance to the farthest object I know ever increases as I am getting to know more new objects. Hence my universe is ever expanding. (The expanding universe fact that have been found by the scientists can also be considered. There, the distance between me and the farthest object is continuously increasing). So we obtained same properties of the well known universe, infinite and expanding. But in a different way.
The universe is different for two persons. Assume the following scenario. You create a fake profile in facebook (let say Geetha), invite me and start to chat. For me, Geetha is a real person and she belongs to the universe. For you, Geetha doesn't exist in the universe. If people I know is another dimension of the universe,(we can define more dimensions in addition to x,y,z,t if we can not define the extra dimension in terms of x,y,z /independence ) it has been expanded with the existence of Geetha.
Though we used the terms "My universe" and "Your universe", only one universe exists. That is my universe or "The universe".
Can we build our theory by continuing in this way ? But we have to answer many questions. Like,
"What happens to the universe when I dead ?" or "How was the universe at my birth ?"
---(image source : wikipedia)