The Milky Way galaxy was 100,000 light-years wide, and possessed—roughly 1,000,000,000 years before 2546—460,311,922,810 stars.
Circa four billions years ago the galaxy developed to the point where life could safely evolve, though it took two-to-three billion years more before intelligent life first created interstellar empires. If unmolested, any one species could have dominated the galaxy in mere tens of millions of years. Unfortunately, the expansion waves of multiple species collided before any one could claim overwhelming control.
Eventually a galaxy-spanning, multi-species war broke out—the Dawn War. At its conclusion only the Inhibitors remained active, and they focused their efforts on inhibiting the development of interstellar civilisations.
Since their rise, no intelligent life had been allowed to expand significantly beyond their birthworlds, until the rise of humanity.
Unfortunately, humanity's rise was followed quickly by the Greenfly, which ripped apart entire solar systems and seemed unstoppable in its course: the destruction of the entire Milky Way.