“they” are our descendants, who have evolved to exist in five dimensions. Because they exist in five dimensions (time being the fourth dimension), their experience of time is not linear in the same way that ours is. They create the wormhole and the tesseract that saves Cooper.


http://www.slate.com/blogs/browbeat/2014/11/07/interstellar_explained_the_ending_who_are_they_the_tesseract_the_blight.html