Let me ask you a question: Have you ever seen the number 3? Some people might answer "of course!" I even just literally showed you the number 3. But did I really? If I'd hit backspace, would the number 3 have been destroyed? Of course not. What you are seeing when I type 3 is a visual representation of an invisible concept.
The number 3, and indeed any number, is an abstract concept. It has no physical existence. You can have 3 things that have a physical existence, but even those 3 things do not make the number 3 physically exist. You cannot destroy the number 3, you cannot contain the number 3, you cannot test the number 3. It cannot be observed by any of our senses, it cannot be detected by any of our equipment, and yet, no one denies the number 3 exists. The number 3's invisible attributes are obvious. If someone doubted the number 3, it would take very little to convince them otherwise, and very much to convince me they are sane if they resisted the idea.
God is certainly not just an abstract concept. However, His invisible attributes certainly are. These, too, are impossible to test, yet also impossible to logically deny. Of course you cannot see God. But why would we expect to? He made the entire universe in just 6 days. If He knows the stars by name, how can we expect Him to be as simple as one? The proof of God's existence is the same as trying to prove the existence of the number 3. Put simply, we all already know He exists.
So why do people still deny Him? Simple: We hate the moral accountability He brings. If there is a God, there are laws we must obey. And how many of us really obey them? I'll tell you the answer: Zero. We're all sinners. That's why Jesus had to die in order for us to be made right with God. But just because God seeks peace with us does not mean we seek peace with Him. Some people would rather remain slaves to their sin. As Christians, our job isn't to convince them God exists, but to persuade them to correctly respond to the knowledge that He does.