Imponderable describes a situation that isn't able to be completely explained. "Where did we come from?" is one of those imponderable questions that defy human's ability for understanding.
If something is ponderable, it is capable of being assessed or weighed; stick im- in front and you get the opposite effect. Use imponderable to describe something that is elusive and vague, perhaps even evasive. When your question doesn't have a definitive answer, you are in imponderable territory: "Life has many imponderable questions, such as why you park in a driveway and drive on parkway."
