Fact: there are approximately 350 billion galaxies in the universe, our Milky Way is just one of them, there are about 250 millions stars in the Milky way and our Sun is just one of them hence there are more stars in the universe then there are grains of sand on Earth and our sun is just one of those grains while our Earth is only half a grain in this magnificent scale of the universe....
When you look at a watch, you know that a watchmaker must have created the watch, when you look at a software program you know that there must also be a programmer....just as everything in this world has a creator, a composer, a maker or an author, it is suffice to say that some super intelligent omnipotent entity must have created the entire universe... this entity is what we call God. Hence the fundamental reason for God's existence is that He is the author, the Composer of everything.....the galaxies the Sun and everything that encompasses the heavens.
After all can u look at a building n say that it was created by itself? That a telescope was created by itself....just as everything has an author.....God is the author of the universe...