Universe means all existing matter and space as a whole. In other words, the Cosmos. It comprises earth and space. Scientists say that the Universe is 15 billion years old. But it is only an approximate estimation. The Stars in the galaxy are 14 to 18 billion years old. Yet it is unclear which is older, whether the Stars or the Universe! Besides, there are some more galaxies, undetected yet, about 13 billion light years away i.e. nearly 6 x 1000,000 x 1000,000 miles. To convert into km multiply by 8/5.
Nobel Laureate Albert Einstein said that the Universe have no centre or an edge. It is the gravity that makes all of space time rather circular. It is reckoned that the mighty explosion, “The Big Bang” only created the Universe. The flash of the explosion (The Big Bang) that appeared in a split second or so, tossed matter flying all around.
The hot ball, so tinier in size cooled down to more than the freezing point, and gradually grew in size as that of a foot ball. Immediately thereafter, an abundant force triggered off the expansion of the Universe to some billion x billion x billion times. It is called the “Inflation.” This vastness of the universe gathered matter and energy. And the enormous force branched off into gravity and electricity.
Clouds are formed millions of years after that due to the clustering of the gaseous substances. The addition of clouds was the result of galaxies. And the billions of stars we see in the sky are considered as the tiny part of the universe. Thus, a chain of chemical reaction led to the present universe, which is yet to be explored by the scientists.
In a nutshell, prior to the Big Bang Explosion, all these material seemed to have been compressed as a small ball. It is believed that the galaxies keep moving even now.
Tailpiece: NASA and other sources have reasons to believe that there could be many more universes about which we knew nothing!