In order to say there is no God, you must know the whole definition of God. And for example you are searching for an item in a several storey building, you don't know where it'd be. You have to check in every nick and corner and turn the building up side down to find the item. Without doing so you can't tell for sure that the item is not in the building.
The same way you can't tell there is no God. Because Science and Technology is not that advance enough to check in every nick and corner of the whole universe.
And If you can see a big smoke far away, You don't need to see the fire to say there is a big fire.
The same way While there are so many signs (a tree, animals, humans, the whole universe) of existence of God. we don't need to see the God to say there's God. And the best sign of existence of one and only God almighty is the Holy Qur'an.