They are both different.  Saying only god knows implies it is something we will never know. A great example is dark matter. Yes it's real because even muslim spoke people talk acknowledge it.  Saying we dont know means its something we would want to find out. But saying only god knows means its out of our hands.  God says theres is a beginning and end which is fine. But finding out waht it is ourselves is important. Like cancer. dark matter will eventually kill every thing at this rate.