A theory is a set of organized statements which attempt to explain something. Darwinian evolution is a scientific theory. That’s because it’s falsifiable. In other words, it can be critiqued; observations can be made and conclusions drawn – based, of course, on the accepted rules of the scientific process. Any science theory must be tested, and the rules of the test phase are that it must be demonstrable, observable, and repeatable.
If the existence of God who created complex life is not acceptable, then a purely naturalistic explanation must be found. This explanation must include all of the basic “turning points”, or key elements of the theory, and if it’s called a “scientific theory”, it must be validated by the rules of the scientific process. The problem with Darwinian theory is that it has been around for a very long time, and the core aspects of the theory have been widely put to the test – and found greatly wanting