That's a huge topic that involves a lot of research and a lot of science. Rather than try and give you a simple explanation which doesn't properly address the issue, what follows is a listing of resources that can give you some glimmering of what's involved.
Some resources are very simple, and some are highly technical and very complex.
We've tried to provide material so that everyone from the layman to the holder of a PhD in the sciences can be challenged. A look at evolution Evolution can be considered to be a theory in crisis. It's not a fact, as so many people like to believe. That may sound bold, but there's evidence to back up this claim. The Second Law of Thermodynamics is often thought to contradict evolution.