I can’t speak for all atheists. There is nothing mandatory for an atheist to believe in. Most of us will agree on some subjects and disagree on a lot more. That is because the only criteria for being an atheist is if the answer to the question “Do you believe in a god or gods?” is NO. Or even I don’t know. If it is the latter than you can’t say that you are a theist anyway.
Alright now to your question. Ofcourse not all atheists will believe in things that can only be scientifically proven. But it is better to believe in things that are proven to be true. Otherwise you could get taken for a ride. Some of them non-proven things are harmless and even fun. In my mind sometimes I am captaining the starship USS Enterprise.
For instance, I believe that there could be alien life out there on various planets and star systems. Is that scientifically proven? No. But I just can’t bring myself to be arrogant enough to think that in this wide, wide universe will millions and millions of planets and moons we are the only planet with life. It would be a shame and waste of space. I highly doubt we will prove it in my life time but it is fun to imagine what life is like out there amongst the stars.
Prompt from Quora.com