Darwin explained all of this with one brilliantly simple idea - natural selection, driving gradual evolution over immensities of geological time. His is a good theory because of the huge ratio of what it explains (all the complexity of life) divided by what it needs to assume (simply the nonrandom survival of hereditary information through many generations). The rival theory to explain the functional complexity of life - creationism - is about as bad a theory as has ever been proposed. What it postulates (an intelligent designer) is even more complex, even more statistically improbable than what it explains. In fact it is such a bad theory it doesn’t deserve to be called a theory at all, and it certainly doesn’t deserve to be taught alongside evolution in science classes.
Hat tip to Diego.