Believe! It is one of the things required to gain salvation freely, as given to us by God. If you have seen the universe and all in it, you will know that there is a Creator greater than all you see with your eye. He does not have to defend Himself because His creations can attest to His power and nature. There are numerous things, seen by the naked eye or not, which can serve as clear evidence of a great God, so that at the end of the day, no one will have an excuse to not believe in Him.