Science isn't really in the business of achieving certainties, such as ruling out the possibility of God. Actually, science can't deal with God at all, as his believers have put him beyond the laws of nature and thereby beyond the scope of science. Science can't be interested in God (which isn't to say that some scientists aren't). What many atheists think, especially those of a scientific frame of mind, is that this is entirely illegitimate, especially as it is not based on even the merest scrap of evidence. All that science can do is point out that the existence of God is suggested entirely on the basis of faith; moreover, he's not a very good competitor with the laws of nature when it comes to creation. You simply can't get away, rationally, with explaining the diversity and complexity of the universe by inventing something that is far more complex itself, that breaks the laws of nature at every turn and for which there is no evidence. Simple as that.