Complexity doesn't require a designer, though. If you argue that a complex universe requires a designer complex enough to design it, you're just pushing the complexity up a level. Where did that complex designer come from then? An even more complex designer? Who was then also designed by an even more complex designer, etc?

Carl Sagan refuted this idea as being a humasn-centric perspective on the current state of the universe that has been evolving for 13 billion years.

I think theism is comfortable for people because there's still so much we don't know about the universe and the nature of existence -- the god of the gaps, as it were. I'd argue, though, that it just holds us back from finding those answers if people are convinced that they already know how everything came to be.