If one proposes that the process took place in a way that would leave no traces, then the theory is not falsifiable. If it is not falsifiable, then it is also not scientifically verifiable. Those who believe in it do so by taking a leap of faith. A second test of falsifiability that Darwin proposed was related to the gradual evolution of organs. The problem with Darwin's theory is that every slight modification of an organism has to give that organism a selective advantage over creatures that don't have the modification.
Organs are like machines that carry out specific functions an organism needs to stay alive.
Consider a mousetrap, a rather simple machine. To work, it needs a base, a spring, a means of attaching the spring to the base, a trigger mechanism and other parts. Unless all the parts are present the mousetrap will not catch mice. Creationists uphold a literal interpretation of Biblical accounts of early events in the history of the earth. Therefore, they propose that the earth is roughly six thousand years old.