It is really quite simple. Whether a god exists or not, is literally IMPOSSIBLE to prove or disprove.
True, but what possible is, is to disprove "known/named" deities, most of which have been disproved thorough history. So even if "a" deity can't be proven nor disproved, "the" deities can and have been. Who believes in Zeus today anyway? Yet many did back in the day and "knew" that he definitely exists, just like today many "know" that JHVH or Allah exist. In 100-200 years, few will believe in JHVH or Allah and new religions&deities will be made up.
There is not a single scientific experiment in which we are capable of to test for a gods existance. That is because a god by definition is SUPERNATURAL. Which means it interacts and exists outside the realms of our reality. Atheists no matter how much they may want or say otherwise cannot scientifically disprove the existance of god. And religious people cannot scientifically prove the existance of god.
In my opinion something being supernatural by definition disproves itself, since supernatural doesn't exist since that which defy the laws of the universe can't exist and that which follows them by definition isn't supernatural but natural. Therefore the only way for deities to exist is if they are natural, in which case they would have to obey the same laws as we do, which would make me question their godhood. Shortly, supernatural is contradictory in itself. If gods and goddesses exist and they created the universe and all the laws of physics that govern it, being omnipotent, can they create a boulder that even for them is impossible to lift? Omnipotence is paradoxical, enough evidence for me against God.
For example, the THEORY of evolution, that all life essentially originated from a single organism, and that animals such as dinosaurs mutated over time to become entirely new species such as birds, could very easily and rationally be explained by a god who chooses to shape the animals over time, by introducing gradual rare benefical mutations.
I notice that you highlighted theory, so I suppose you may be one of the many confused. The world theory doesn't have the same definition in scientific usage as it has in common talk. In common talk, theory is often used to mean a guess or a hypothesis. While in science a theory is an explanation for phenomena supported by evidence. If there's lots of evidence for a theory, it's no different from a fact. So a theory of evolution are a set of explanations for the phenomenon of evolution, which isn't just maybe, it's a known and observable fact, there's literary zero doubt that evolution is true.
The fact that the earth just so happens to be in a perfect place around the sun, and that the sun happens to be a perfect age, and perfect size to make conditions extraordinarily well suited for life, and that the earth just so happened to have the right chemical and elemental make up(essentially vast series of coincidences.)
So one could look at just how all the universe functions, and see a evidence for a god.
That's a flawed way to look at things. The majority of the known universe is hostile if not downright deadly and chaotic, that's why in those places there can't be life. The fact that we exist is simply because there happens to be an ideal place to sustain life, not because an ideal place to sustain life was created by God for us so we may exist. There's no intent or purpose, we exist not because of prestige but because if it were any other way, we wouldn't be having this discussion.
Then you have the personal interaction with God. You have people who had literal conversations with God. You have God acting in supernatural ways, both big and small, with his creation. You have people who experience miracles and the such.
Simply though, I believe there is a God. I have witnessed things that should not have happened. I have seen way to many personal coincidences in my life. I have seen how he works in the lives of others. See I believe in the God of the Bible. But to talk about those things, is for a different discussion. I have my opinion and you have yours.