The Universe is everything; everything around us and within us. So, there are living things (not only those living on Earth, but all those living things that we do not know whether existing in the planets of Sun or of other stars), stars and their planets with satellites, galaxies, smaller subatomic particles, and all matters and energy. Wow! So vast and huge, am I right? Yes it is. The observable Universe is about 91 billion light years or 28 billion parsecs in diameter. It is only the observable size of it, but the actual size of it is not known. We know only a little about the universe including its size. Is it having a finite size with clear boundaries or is it infinite with no boundaries? We do not know.
The Universe was never the same as before by size. It all began after a Big Bang. Not me, the scientists say so. Before the Big Bang, there was no space and no time. The Big Bang theory says that it expanded from a dense and hot state and it is and will continue to expand.
Neither we nor the scientists are so sure about what had happened during the initial 300,000 years of the Universe. After many million years, the dense areas pulled in material as they had higher gravity. Almost after 100 million years of Big Bang, the gas turned hot and dense to form the early stars. New stars took birth in a faster rate and large groups of stars formed galaxies. The earlier galaxies were smaller and nearer causing collisions.
The Universe has billions of galaxies or better to say indefinite galaxies as we have not observed the whole of it. These galaxies do have many stars and their planets with satellites. Our Solar System is one among the many stars of Milky Way Galaxy. It is also interesting to learn about galaxies and Milky Way. There is a large empty space between the galaxies and between the stars.
So, the Universe is everything that we can see, touch, feel, measure, or sense.