If God had wanted to prove himself to the world he could simply appear on the sky. If he did that everyone would be struck by fear and then it would no longer be free will. Also this isn't just about proof it's also an issue of the heart. If someone does not want to believe nothing will ever convince him. I heard many testimonies of scientists who came to faith in God and started with own research, some even trying to disprove God. Research and facts can point a person into the right direction but I don't think that facts can ever be enough. In the end it always comes down to faith and making a decision.