It is reasonable to believe that there is a God who created the universe because there is no way to prove there is no god and because there is no scientific way to explain why the universe exists. If there is no scientific way to explain why the universe exists, it becomes reasonable to postulate that a supreme being created it. However, it is also reasonable not to believe in God because there is no proof that a god exists.
There is, of course, a scientific explanation for how the universe has come to be the way it is. This is the Big Bang Theory. This theory holds that the universe began as an infinitely dense fireball that exploded, leading eventually to the universe that we now have. Even if this is what happened, it does not preclude belief in God. There is no way that science can say why or how the infinitely dense fireball came to exist. This is where religion comes in.
The reason for the existence of the universe is a mystery. Since it is a mystery, it is reasonable for people to try to explain it. The existence of God is one way to explain the presence of the universe. Therefore, it is reasonable to believe that God exists.