Two very different things: why does the universe exist and how did the universe come to exist. There is no why for the universe. It is. Looking for a why is what theists do.
Clarifying the clarification: I was only attempting to answer the question, "Why do atheists flock to science?" My answer was simply that, rather than arguing something vacuous, they simply say, "Meh, the scientific explanation will do for me." That is hardly flocking but just throwing a "good enough" explanation back at the theist that they will hopefully leave us alone.