Would the question be, "Why do I exist?"
Or the question could be, "Why does the universe (or anything) exist?"
Or, "What is the nature of the universe that exists (or seems to exist)?"
I am going to go with, "What am I?'
I will also go with, "The only things I know (or think I know) are what are in my head."
Further I don't know what I don't know, and, to an unknown extent, I have no idea about what I don't know.
So, let's get back to what's in my head
What's in my head is a result of a brain that has grown (developed), a set of lived experiences, and a ton of data input that did not originate with me.
While my brain experienced sensory input in the months before I was born and immediately upon my birth, my brain at the time was not developed the way it is now and was not at the time equipped with language, thinking abilities, and consciousness of memory.
I don't purport to vouch 100% for the development that came about. I report I have no memory of having a developed brain when I was born, I have a developed brain now, and I have recollection and data indicating there was development over time.
The concept of time looms large. That concept is in my head, and I take it as something basic, even though I cannot be totally sure about it.
Also, I have a strong belief there are other brains in the world, and my brain is not the only brain. Further, data input into my head and lived experience has created belief that people are born, live, and die, and many people (with brains in them) have come before me.
The data input I get from history and science books includes that previously the Earth was believed to be the center of the universe and the Earth was previously believed to be only a couple of thousand years old.
Since then "scientific" knowledge has grown enormously in what it purports to say is the "truth" and "reality"about the universe, and it says that the universe is 14 billion years old (and had some sort of creation) and the Earth is an infinitesimal speck in the universe.
[It is worth your tracking along with me here. Can you go with "What am I?" as a basic question for you? If you can go with it, can you go with what I have set out above to start my answer to the question? Would you diverge from what I have said? Where would you diverge, and what would be your divergence?]
[Now, you may diverge quickly based on your religious beliefs. If you are a Christian, your religion gives a fulsome answer to the question of what you are, and what I have started to set out is not going to meld with your answer. I don't know if that means you can and will terminate tracking along with me here.]7/6/18
I am bogged down contemplating religious belief versus science.
I don't think you can have it both ways. Science is dead set against the Christian Biblical God. Scientists may be formulating ideas about a creator of the universe that did the creation science now believes in. I have no idea what those ideas are, and, for my operating purposes, I just don't have any idea about such creator. (Please tell me, though, if you disagree.)
Back to what is in my head: nothing in my head gives me any sense that there was anything of me for the 14 billion years that preceded my birth, i.e., I was non-existent, a nullity, during such time. Contemplating that non-existence before I was born leads me to contemplate that I will have the same non-existence after I am dead.
Just about everything argues against the Christian Biblical God, so I give no credence to the suggestions put forth by the Christian Biblical God as to my condition after I am dead.
Many have tried to make and argue the case about the Christian Biblical God on both sides. I am sure I cannot add anything to the case against the Christian Biblical God that has already been made many times.
So, then, where in the hell is this disquisition going?
