There are some ideas that the big bang is like a cycle, it starts, the universe expands, then collapses, and then starts again, and so on.
It could also be a once-only event. But how it was made is a tricky question because you would need to know what was before the big bang.
It turns out that this question by definition cannot really be answered. Why? Because the big bang started time itself, so there was no time before it. In fact, there was no before.
This is much like going to the South Pole and standing exactly there. Everywhere else on Earth, you could ask the question: what is more south than where I am? The answer would be the South Pole. But if you are standing exactly at the location of the South Pole, then what is the answer to “what is more south than where I am”?
There are lots of different theories about how the big bang happened, but we don’t really know for sure. Some astrophysicists study something called the ‘cosmic background radiation’ which they think is the remanents of the big bang. This might be able to give them some clues as to how the universe was made.
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