How could matter, in an atheistic paradigm, account for an objective moral law?

Just ask the question and draw out the answer.  There is no answer to this question that makes sense.  They may site some moral systems that don't mention God, but that does not mean that the moral systems are objective.  They may be absolutely believed, but they are arbitrary.  If an unbeliever says that something is right or that something is wrong, they either must reference God or else be illogical and arbitrary.