If i read the eldar fluf right, the webway used to be something the eldar could modify at will. As the webway is a warp passage that stay in the real world i assume the warp extend as long as there is real world. Judging from the fact tht tyranide have come from an other galaxy i think it's safe to assume the warp and the webway might extend a very little bit further away from the galaxy itself, at least as a left over, left behind by the coming tyranide don't you think?
"Modify at will" leaves a lot of room for interpretation. As far as I know, there's never been any statement to suggest that the Webway goes beyond the galaxy.
There is also the fact that necron went to war outside the galaxy while some of theme stayed asleep if i'm not wrong. For some reason i don't remember, they decided to come back, that might have been helped by the fact that they encountered something on there way out. I don't know. Or they had trouble travelling without the warp and the webway.
The Necrons fluff has changed a lot through the years, but as far as I know, they've always been confined to the galaxy. Maybe some splinter force?
EDIT: Indeed, seems like the Silent King travelled intergalactic space and saw Tyranids out there. He's since returned.
If tyranide run away from something even worse... what will that be? Because if they flee when tyranide have such a strong will to advance and conquere, if all the intelectual race don't fight together, I think it's going to be a massacre.
It could just be speculation by Imperial or Eldar intellectuals. Even if they are fleeing from something, GW will probably not reveal anything in the foreseeable future (if ever), and merely use it as a storytelling device.
Ork signal... what ? Where does ork comes from btw? As far as i remember they have the most powerfull god might have something to do with what the tyranide flee.
Last time I checked the fluff, the Orks were created by the Old Ones (who called them the Krork) to fight the Necron(tyr)s and Enslavers, as a kind of biological weapon. The Old Ones implanted them with various abilities, such as the Waaagh! gestalt psychic field, and a genetic memory of various technical blueprints. Eventually the Old Ones died off (or escaped to somewhere else) and so the Orks are running around willy nilly these days.
I know the 'nids come from outside the galaxy (and there's rumors that they're running away from something even worse), and then there's whoever or whatever made them (they have to be some sort of bio weapon gone terribly wrong...or terribly right). If the Warp focuses around living beings though then there's got to an 'edge' in the warp where the galaxy stops.
I've always hoped they're just a highly succesful result of natural selection. There's a couple of reasons for that, one being that I feel there are more than enough experiments gone wrong running around the 40k universe as is, and the second is that I generally dislike the tendency of GW to have to insert simplistic explanations along the "a wizard did it" (or in this case, "an ancient species made them"). There's enough "agency" around, imho. What I'd like is a compelling dive into a hypothetical track of evolution in a universe where psychic powers are real.
But then again I had hoped they would've used the Necrons to explore ideas within how AIs (or AI-ified beings) perceive the world, process information and operate in groups. But then they went with Robot Space Egyptians. Eh, fair enough.