One of the more frustrating times in my discussions, is when I'm expected to defend a position of non-belief. I have been accused (several times) that the atheist position does need to defend itself, and claiming it doesn't because atheism is a response to a truth claim, not a truth claim in itself, is disingenuous. It doesn't seem at all contradictory to my religious friends that there's no need to defend against not believing in unicorns, mermaids, or fairies. The reason why there's no need to make a defense for non-belief of a God is for the exact same reason that Christians don't need to make a defense for not believing in mythical creatures.
When you make a truth claim, if you have any desire to convince others, then you have to provide evidence for that claim. If you don't provide evidence, or the evidence isn't convincing, the person who is not convinced doesn't need to provide evidence for not being convinced. Granted sometimes people ignore evidence, or maybe the evidence they're presented with is too confusing for them, or they distrust the conclusions others have drawn from the evidence. It sometimes can be frustrating when dealing with people that, for political reasons, or ideological reasons, deny the incredible amount of evidence for evolution or climate change. You may be completely taken aback the first time you realize someone is willing to disregard so much scientific information, because they are unwilling to question their worldview. With all that in mind, I still stand by the idea that you don't have to defend non-belief.
If, however, you are going to substitute one belief for another, creationism over big bang cosmology, for example, then you are making a truth claim, and you are in a position that would require a defense. Atheism, in itself, isn't substituting one belief for another. I might say, "I am not convinced there is a God." "Then how do you explain how everything came to exist?", someone might respond. I would probably reply with, "I don't know. I have read a lot of different ideas on the topic, but, truth be told, I don't think there's enough evidence to really know for sure." That's it's. That's the end of the conversation. "I don't know." is really the best response when you don't really know something. You don't have to make up an answer, you don't have to choose from a list of other beliefs. You can simply say, "I don't know" and leave it at that. Maybe one day we will be able to answer that question, maybe not, but until that time, saying, "I don't know" is the most honest answer anyone can give. But, even if we do answer that question, I guarantee there will be a thousand other questions we still don't have an answer to, and "I don't know" will still be the best answer for those questions.
Does that mean that atheists don't believe in anything? Absolutely not. I, myself, have several beliefs, and I'm willing to defend them. But, I'm also willing to change my beliefs if someone can convince me that I'm wrong in one thing or another. The thing is you're never going to know what an atheist's beliefs are until you ask them. There's no way to know in advance. Atheists aren't monolithic like that. Atheists can be on any political side, they can have beliefs in an afterlife, or they may even reject science all together.
Atheists do tend to gravitate to similar ideas and beliefs. But, atheists don't have a creed or doctrine that we must follow. It's not a church or another belief system. Although, atheists do get accused of being another type of religion all the time, but I've never heard it said better than, if atheism is a type of religion, then bald is a type of hair color.