A simple answer to this would be that it is not necessarily true to say that what we cannot see does not exist. Limiting the world to just our five senses is an escapism attitude which simply brushes away the possibility of whether God does or does not exist without further inquiry, and thus deceiving oneself. We cannot see God because God did not equip us with the senses to “see” Him.
As an analogy, let us say that I have created a car. Is the car able to see me? The car is unable to see me because I have not created the car with the ability to see me. Just because the car cannot see me, does that mean I do not exist?
Despite the car not being able to see me, I can, however, see the car!
Similarly, God can see us in spite of our inability to see God in His physical form. The very fact that the car exists also proves that I (the maker of the car) also exist.
Similarly, the existence of our universe is also a fact and a testimony to the existence of its Creator.