I'd like to avoid discussing the reason for god's existence, because there's already a good thread for that.
As far as I understand, a god is an elderly male person with a white beard who posses unlimited energy source, that can create a universe, and shape elements to his will. He can also create invisible living beings, whom he puts into organic robots to experience the universe, but also be a slave to his will, or else, when those living beings leave their organic robots, they'll experience burning and torture for unlimited time.
When I was still an atheist, I was debating with another atheist, who insisted that all atheists should take the stance that it is impossible to answer the question "do you believe in god" because the term "god" does not have a definition (something that can be tested via the scientific method).
I disagreed with him, and eventually came up with a definition of "intelligent entity capable of creating the laws of physics". The intelligence part could be measured with e.g. an IQ test. Demonstrating the creation of the laws of physics would not actually be provable beyond doubt, but e.g. a video showing the creation of the laws of the physics could potentially be provided as evidence for us to ponder. He wasn't able to fault this definition as "incomprehensible", but he still didn't change his stance.
Since then, I realized that there was a perfectly good model for the universe - that of a computer simulation, with God as the computer programmer (or maybe operator). Just the same as when you play a game of Pacman, except the Pacmen have artificial intelligence. You are god to them. You created (by firing up the program) their universe, and you can destroy their universe at any time (by powering down your computer). Depending on your debugging tools, you can even zap the computer memory in real time, e.g. perhaps changing Pacman's colour from yellow to red. God could well be a snotty 10 year old kid.
This provides an explanation as to where God exists (different dimension) and why you can't be sure what he looks like, but the bible verse "God made us in his own image" makes a hell of a lot of sense. ie the 10 year old kid looks like us, rather than being e.g. a giant intelligent cockroach-like creature.
I would also hazard a guess that God himself is implemented in silicon (or some equivalent), so asking what he physically looks like doesn't make a great deal of sense, since, just like Pacman, he can look like anything he or you wants. It's just a matter of rearranging a few bytes of computer code.
Computer code is extremely flexible. A better question to ask is - what sort of God and universe would you like to live in for eternity?