Board Thread:Warhammer 40k General Discussion/@comment-192.138.62.36-20140116164341/@comment-6078851-20140214131730

it's explicitly stated in numerous canon sources, their codex being one, that tyranids are intergalactic travellers. tyranids are most certainly dormant, mostly, in the void and are slowly waking up, but it's part of their continual drive to survive - wake up, devour galaxy, head towards next galaxy, go back to sleep, wake up, devour galaxy....etc.

tyranids first reached the milky way en-masse around 250 years ago, approx 750 m41, but tyranids have been in this galaxy, technically, for quite a bit longer - genestealers are sent ahead, kinda like scouts with homing beacons, and reach a galaxy long before the rest of the race - like a few millenia before. if they survive and thrive then the hive mind knows that said galaxy will have bio-mass to plunder. it wasn't until tyranids arrived en-masse that the imperium discovered that genestealers are tyranids.