Suppose we have a mass $m$ in space, and we then introduce a bigger mass $M$. both have no kinetic energy initially.
$M$ & $m$ attract, and move towards each other gaining some kinetic energies along the way. At this point in time, the overall energy of the system isn't 0.
Since Law of conservation of energy will always hold up, there is obviously some flaw here that I've overlooked.
What is that flaw?
