Long before I asked this question, great many thinkers of the yore asked themselves the same question, and each came up with their own answer.
Among these, the answer that most satisfied me was the one given by the great Indian philosopher, Sankara, who probably lived in the 9th century A. D.
His unequivocal answer is, "You are It!"
Modern astronomy tells us that we are all made out of stardust, or cosmic dust. This cosmic dust came from the "Original Thing", the "It", or whatever it is, that exploded with a Big Bang some fifteen billion years ago, and is still expanding. The Sanskrit word for the thing that expands and fills the entire space is "Vishnu." So the Big Bang is "Vishnu's lila", a paly, a game.
So, I am Him. And I am part of His play.
At a more mundane level.......
Who am I? I - My memories, My sense of personal identity, - am no more than the collective behavior of a vast assembly of neurons.
I came to the U. S. in 1961 from India . I grew up in Tuni, (a small town in Andhra Pradesh ). Tuni is located on the main railway line between Madras and Calcutta on the east coast of India, about 100 Km south of Visakhapatnam. People say that Tuni made its name (a) as the best "meals halt" on the Madras-Calcutta rail line, and (b) as the grower of the best "betel leaves" in all of India.
If you ask me, Tuni has better claims. On the outskirts of the village there is a small natural body of water called, Lakshmi Devi Cheruvu. On the banks of this tank, in my childhood, there were huge masonry cisterns. We called them “Neeli Mandu Kundeelu”. These were the remnants of an indigo factory. Indigo, as you know, is a blue dye extracted from some native plants. When the Germans began to synthesize indigo, the factory went bankrupt overnight, literally. My interest in science, in particular - chemistry, started when I first heard this anecdote.