Many theists like to throw out the Big Band, consciousness, epistemology, and abiogenesis as 'untestable and unprovable' areas of science. The implication is that, since we don't know exactly how they happened, there must be some higher power. That implication is based on an a priori set of assumptions involving the existence of a deity. An atheist does not share that set of assumptions, and sees the world (including the above phenomena) in a very different light.
Even though the Big Bang is currently a little beyond the depth of my knowledge, I'm not going to sit back and attribute it to a mystical force. I especially take issue with the fact that Christians assume that their God is the one to explain these phenomena - even if the Big Bang were completely inexplicable, it still would not invalidate Biblical contradictions, and therefore would not give me reason to believe in the Christian god. To make a long story short, just because I don't know doesn't mean a deity exists.
Religion needs to be kept out of science classrooms because it is fundamentally at odds with scientific inquiry. To attribute the earliest moment in time to a god is to think you have the ultimate answer - which stifles further inquiry into the subject. Most theists have a remarkable ability to incorporate scientific findings into their views just fine - and as long as they don't mistake their faith for science, I have no problem with that. It's when reactionaries decide that they don't like the consequences (imaginary or real) of a theory like evolution that theists are given a bad name.