Nothing is a perfect fit in the world, because nothing is perfect (sorry, Theists, deny it all you want but personally I don't see ANY reason to believe in something that cannot be seen in any shape or form (even the tiniest of molecules can be seen under some form of microscope and are proven to existed) and the fact of the matter is under no microscope or eyesight can God be seen. If you say God is in your head, I suggest your dedicate your life to proving God's existence well and truly by letting a Scientist cut open your head and use modern technology to search for God. It may just be my personal opinion, but I see no reason to believe in something that cannot be seen.
So as I said, nothing is a perfect fit, but the closest theory we have to a perfect fit is Evolution. Athleticism is a good example of this. Athletes get stronger and quicker every generation. Usain Bolt's record WILL be broken in this generation or the next, and so on and so forth. NaturalMotion, a game development and scientific research company used a supercomputer to track the growth of computer designed figures through generations, I believe using mathematical equations related to the theory of evolution. They started off just falling over straight away, without the ability to walk. From generation to generation this came to stumbling and walking like a toddler, still falling over, to being able to walk properly and confidently like we do, to being able to jump long distances and climb/run/walk faster and easier. This is similair to what has happened in real life. Thus, you could already argue there is more evidence pointing towards the theory of evolution than there is pointing to God in that one study carried out.
The fact is that the theory of Evolution is more logical than ANYTHING Theists have put out to defend themselves, and at this stage it is very hard to argue against. Returning to my point about not being able to see God, Evolution has been clearly visible for many years, whereas God has never been.
I'm incredibly ignorant in the subject, I have not been given the chance to do philosophy as an IGCSE so my knowledge goes from watching documentaries and reading books and things like the God Delusion. However, I'm aware that I'm nowhere near as ignorant as most religious people nowadays.
One well known Biologist, who's name I cannot remember, was recently slated quite publicly by Dawkins for saying "all the evidence points towards evolution, but I will carry on believing in God because my religious scripture says I should". Theists, does this not tell you everything you need to know?