The universe is currently expanding. That is known. It was once much hotter and denser. That is known. The rate of acceleration is increasing. That is known. It was once hot enough and dense enough for nuclear fusion to be present *everywhere*. That is known.<quoted text>
big bang started universe...NASA won noble prize in physics for it...
What is *not* known is whether the beginning of the current expansion phase is the beginning of 'everything'. It is certainly possible. But it is also possible it was a transition of some sort. This gets into a bit of terminology. We tend to say 'our universe' for the current expansion phase. if there is more than this, then the whole is often called the 'multiverse'.
For example, it is possible that there was a previous contraction phase before the current expansion phase. It is also possible that our universe 'budded off' from a larger, eternally expanding, multiverse via a quantum fluctuation.
So the Big Bang is proven: the universe was once in a very hot, dense state where nuclear fusion happened everywhere. It has since expanded and cooled.
Whether the beginning of the expansion is the beginning of 'everything' is NOT known. We'd love to have some data to determine which possibility is correct.