Machu Picchu. Who made it?

Hi followers! Last year, I went to Cuzco and visited Machu Picchu.
Machu Picchu is one of the seven wonders of the world. Moreover, the road to get to Machu Picchu is dangerous, so you'd better go with an expert guide. You could go with your friends, too. If you've never gone to Machu Picchu, you have to visit it as soon as possible. Well, I'll continue with my story. Since thay day, I've been wondering who built Machu Picchu, but I haven't found concret results. My parents say that I shouldn't waste time trying to solve this stupid mystery, and that I'm supposed to study physics, instead. Even though, I still searching, and some of the things I've found are here:
Some people think that it may have been built by aliens. There're pictures taken at Machu Picchu that shows shadows of extrange things, and people say that they may be UFOs. For me, it's illogical, it couldn't have been built by aliens. You ought to visit a psychologist if you believe in those things.
Others say that Machu Picchu could've been built by Incas, and that they had an amazing strenght. It may be true, but what did Incas do to get that strenght? Well, in fact it was possible because they had a healty diet. They could've prepare their food with native products.
At the end, I got more confused than I had been when I started my research. Shall I stop searching? I don't think so.. I'll have to do everything to solve that problem.