There are all kinds of theories. The Big Bang Theory, the steady-state theory, the God theory, the back of the turtle theory. All are valid or invalid depending on your intelligence level.
As a physicist I have spent a lot of time contemplating it all and have come up with the final explanation. It is really quite simple. So simple you would have never thought of it and by definition it would be impossible for anyone except me to be able to comprehend the real facts.
The simple truth is that the universe began at about 5:30 am on May 13th 46 years ago in what you would consider to be 1968. Our universe is a part of the Multiverse where billions of other universe bubbles float around and bang into each other. Our particular universe sprang into being when two of those other bubbles bounced off each other.
Not really a bang. More of just a sudden reality. The random formation of elements came together basically as you see it now. Nothing existed prior to 1968. You may think that time existed before then but it is merely an example of pre-configured history. To you and everyone else, those memories and written history seem perfectly real as if they actually happened but they in reality they just suddenly appeared when the universe began in 1968.
Not only can I tell you when the universe began, I can also predict the approximate time of its end. We are currently past the peak of the curve. The universe has already started decaying. The rise of entropy level is obvious. Just look around you. Our existence has, at most, 40 more years left. At that point everything you know will quickly evaporate and return to the nothingness from which it sprang.
Fortunately there are plenty of other universes appearing and disappearing all the time. When ours is gone the emptiness it leaves will soon be filled with a new one.
Easy peasy. Case closed.