The Fuck?

(HL1 Part 40) I'm not sure what's going on here, but someone knows something they aren't talking about. How do the scientists at Black Mesa already know about the borderworld Xen? Why are the inhabitants of the borderworld pouring into our world? There are humans that have been to Xen already, that are working on scientific experiments and whatnot, why? Too many questions, not enough answers for Gordon.