It's not a contradiction - it takes a lot of faith to be a dedicated atheist.
The origin of 'everything', including life, is and always will be an unknown. We may develop models for the origin of the universe; they may explain everything we can see. But they may not be correct.
Look at Sir Issac Newton. He developed what is called 'Newtonian Mechanics', which explained everything that could be measured and observed in the 17th century. It worked very, very well, and engineers use it to this day - because at the scale at which the observed world works, it's a good model.
But it's fundamentally wrong. When you go to the very big and very fast, or very small, the Newtonian model breaks down completely. And since it can't explain everything - it's fundamentally wrong, when looked upon as a template for 'how the world works'. It's a partial answer...kind of a physics band-aid.
Newton didn't have all the facts. He didn't think on the scales where his model would stop working. Not his fault - he was limited by restrictions in what he could observe.
So it is with anything that tries to explain everything. We can only see what we can see. If we look at the Big Bang Theory, we can 'imagine' the conditions inside the singularity that, smaller than a pinpoint, contained all matter and energy.
But what was outside that pinpoint? Nothing? What is nothing?
All of these questions require some degree of faith to answer, and to live with. And that faith can be just as deep as that of a religious person who believes that God moved upon the face of the water.