DNA exchange

It seems that I keep running into evolution problems. Maybe I already knew the majority of the problems with the creationist theory and so haven't come across any new ones. This new problem is with DNA. According to Darwin's theory, living things started with a common ancestor and then branched out as they evolved into different paths. A problem with this is that there is identical DNA sections found in different "branches" that did not exist in the precursor. This occurs in bacteria, plants, and animals. One of the theories on how this happened is called horizontal gene transfer. The idea is that genes were transferred from one species to another species using a variety of mechanisms. I'm not an expert in this field but, while I have heard of gene transfer in single cell organisms, I have not heard of it happening naturally in plants and animals. This may be a bit off the main track but aliens as creators seems to be the most logical solution in my mind right now. It solves the issues with evolution and God. I wonder why this theory is not more prevalent?