I have been listening to The Long Earth series of books by Terry Pratchett and Steven Baxter. The title Long Earth comes from a discovery called "stepping" which is the ability for people to move between parallel works. Most people need a "stepper box" to do the stepping, but some people are born to be able to step. This stepping can happen across thousand and even millions of parallel worlds.
While stepping across these parallel worlds humans discover other intelligent being that are natural steppers and one of these beings are what humans call "trolls". These trolls are described as large bipedal primates that are covered with thick hair and have the intelligence of a chimpanzee. They communicate by singing songs and are not aggressive. They will work with humans as long are there are not more then 890 human in the area. These trolls have lived a stepping life longer then humans and can easily move between parallel worlds.
As I was reading about the trolls in The Long Earth it occurred to me that many of the features of the trolls fits the description of the Yeti/Big Foot. The ability for trolls to step also answers the question about why Big Foot is so elusive and seem to disappear when spotted. Trolls and Yeti are the same being.
Now to further blow your mind. Since now that we know that the Yeti is the Troll described in The Long Earth series of books, stepping must be real. And, if stepping is real then the Long Earth must be real which means that string theory is correct and there are multiple universes. Now all we need to do is build stepping boxes.