Question: What is the one unanswered science question that you wish you knew the answer to?

Arttu Rajantie answered on 22 Jun 2011:

I am not sure if this is a science question, but I would like to understand why the universe exists (or perhaps more precisely, what it actually means to say that it exists). It really is the ultimate question we cosmologists try to understand, but in practice there are lots of more concrete questions which we have to answer first, and which luckily we can answer.