Personally I think it's just a fan theory that's never going to be anywhere near canon but until we hear otherwise some people like playing with the idea of it and I've seen a couple of Galra-Keiths floating around BS if you're interested in exploring that angle.
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but here you go. I went tumblr diving and found this and it seems to encompass things pretty well as far as I can tell.
http://autumn-sweet-fae.tumblr.com/post/147280614564/so-what-is-this-keith-alien-theory-it-started
Personally I think it's just a fan theory that's never going to be anywhere near canon but until we hear otherwise some people like playing with the idea of it and I've seen a couple of Galra-Keiths floating around BS if you're interested in exploring that angle.