>>96894314
>The Imperium is the dominant power in the galaxy
No. The Necrons are the dominant power in the galaxy, so much so that the Imperium is largely beneath notice, and even their regional warlords are able to throw back Imperial crusades lead by primarchs without much effort.
The Imperium, of course, is fairly naive, so they end up making the same mistake the Tau do. They think that because the Necrons haven't wiped them out yet, that means they can't, when in reality the Imperium is just too weak to be worthy of their notice.