First of all, as others have already pointed out, evolution may not be a "reasonable explanation" for you, but it is for other people who have different belief systems. Just like the bible or a rainbow aren't "reasonable explanations" of the existence of deities for me. If you want to learn about other people's beliefs, you can't dismiss them before you have even given them a chance to respond. That is why you have a lot of people giving you crap on this question. The question is not offensive, but the way you have asked it is.
Secondly, there are lots of places around here where Atheists and people of other religions have discussed this topic respectfully. Here are some links to answers, forums and articles from the Atheism spot that you might want to take a look at:
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(we also have a couple of 'Ask A Christian' forms)
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If you look through any of those links you'll see that Atheists believe in a wide range of things for a wide range of reasons. We, for the most part, don't mind explaining our beliefs to people who want to learn about them, but we do require respectful debate.