<< If matter and energy have always existed, then how can any concept of time be reckoned? At what point did the big bang "happen", and if matter always existed, why didn't the bang happen sooner? >>
Simple: before the big bang, there was no time-dimension, so when the big bang happened, time started because the time-dimension was formed. There is no 'time' before the big bang.
<< If we believe in cause and effect, then what is the cause of that motion? >>
Why should there be a cause? In Quantum Mechanics, causality works totally different compared to on a larger scale, like our world. Basically it comes down to that something can occur without cause, like a particle which is formed and disappears 'out of thin air'. This is a reason why a perfect vacuum is impossible, there will always particles be formed and decay in it. Like such a particle can just 'pop up' in a vacuum, the big bang could have occurred.