Assumptions are a mortal sin in science... The big bang theory is based on many facts... The reason why it's unproven is because it's only highly plausible it went that way, but it's hard to prove. This goes however further than just a hypothesis. When something is only a hypothesis it would be the "Big Bang Hypothesis" and not the "Big Bang Theory"... When the Big Bang is proven the word "theory" should also be removed.
And I must thank Octavia to confriming Darwin never studied the universe, or at least not in a professional matter. Darwin was a biologist, not an astronomer. Those are very different branches of science. Oh, and the evolution theory has by the way been proven. It's just that some examples based on this theory were proven wrong, but that didn't debunk the theory itself. But Darwin's theory does not explain any origin, you can explain our origin has humans and of other living things by trying to go back in time, with this theory in your mind... But Darwin's theory only affects living beings... Not the universe itself. Darwin also never assumed things... His theory too was based on facts he noticed.
Please note, the creation of the universe by God, or by any entity like God depending on your religious background, was an assumption. Everything they couldn't explain was assumed to be the work of a god, but facts and evidence was never provided. No facts have ever been provided. The Big Bang theory came to be by knowing more than the people in the past did and turning everything we know into the most plausible outcome. It is still possible the Big Bang theory is wrong, yes, but still it's not just an assumption. It's typical how religious people blame scientists for assumptions while what they believe was an assumption from the start... Pot calling the kettle black... Sorry for putting it so bluntly. I'm not an atheist, but when it comes to evidence God exists, I can provide none... Nobody can, religious people CLAIM they can... scientists on the other hand do provide evidence on which theories are based. That does not fully prove the theory (since not all piece of evidence in a criminal case prove the suspect is guilty either), yet this evidence does make things likely. they only need more evidence to deem it fully proven, and that is what they are trying to find with their investigations. Religious people don't investigate. The Bible states God created heaven and earth, and nobody questions that... Speaking of assumptions...