Species 8472 lived alone in another realm/Galaxy called fluidic space.
the Borg Invaded it and wanted to assimilate them because they are the Apex of Biology.
they Failed and ended up getting their asses kicked,8472 entered the milky-way to defend themselves and to destroy all those other species willing to pollute their realm.
so get your facts straight, people just wanna bash Voyager at every opportunity they get.
In the Flesh,describes species 8472 as more cunning and preparing to assault the Alpha Quadrant while the Dominion war was about to end.