The ancient alien astronaut theory is one of my most favorite theories. I did the first writing on it and I will do the others on this topic as well. The main idea of this theory is that extraterrestrials have visited and interacted with humans in the past. These "aliens" may have lent their advanced technology and knowledge to help build temples, pyramids, walls, etc.
There are thousands of unexplained things, and I am a huge fan of this theory's take on all of the coincidences or unexplainable structures. Keeping an open mind to any possibility is essential to the ancient alien astronaut theory. These ideas are not proven and they may never be, but, they also haven't been disproved. It is just one group of people's opinions and ideas about god or gods, the unexplainable structures, and coincidences in time.
The ancient alien astronaut theory is very out there and quite interesting. There are many scholars that believe this theory and perform their own research. It is not all just conspiracy theory nut cases. Some of these people are well respected professors. One day, I would love to be able to end up in this field of study; although I have no idea how to even begin.