I looked for many websites, and I just put one of them. The universe is a very abstract theory, and even the scientists know it very few.
The scientists believe our universe begins by a "big bang", and it is 13.73±0.12 billion years old. It is composed by the space and time, and includes all kind of matter and energy. It is the expanding space as we know. Its definition is everything that existed before, exists now and will exist. We are in the galaxy Milky Way, and there are maybe more than 100billion galaxies in the universe. Small galaxy includes 10 million stars, and the giant one includes one trillion stars!
The scientists also discuss that universe has a limit or infinite. However, we can't see the whole universe. It concerns about the general relati, and that is another complicated theory.