People who study the universe and how it began are called cosmologists. Most cosmologists think the Big Bang theory is the best explanation we have at the moment for how the universe came to be like it is.
Once upon a time... No! That won't do. We're trying to think about what 'was' before there was any time. Let's try again.
Somewhere between 15 billion and 8 billion years ago (but 13 billion years ago is the best guess) there wasn't any time or space. Everything in the universe that didn't exist yet was squeezed into something so tiny that it didn't take up any room at all.
No one knows how. But we have a pretty good idea of what happened 'next'...
Most of the really important things happened well before the first tiniest fraction of a second had passed. Because there was so much energy in the universe it got too hot--about ten million trillion trillion degrees! The pressure was unbelievably enormous. So it exploded.
But the smaller than a pin prick universe didn't really get bigger. It wasn't exploding into anything because it already was everything.
Before we go on, let's check out what happened in that first, tiny fraction of a second.
By the time the Universe was 300 000 years old it was much cooler and most atoms had been made. The universe was smooth. But not perfectly smooth. There were just enough lumpy bits so that when it grew older (and 'bigger') clouds of gas could form. As the universe expanded and cooled, first galaxies formed...and, eventually, stars.
We only found out the universe is expanding less than 100 years ago. It was a remarkable idea at the time. Even Einstein didn't want to accept it at first. But from all that work we're now fairly sure that the Big Bang did occur roughly about 12 or 13 billion years ago.
Frankly, I find it mind-boggling.