There may be a peaceful Alien race that is simply so powerful, it can`t be defeated by other powerful races, yet it has no wish to overwhelm others simply because it is not `made` that way. Perhaps it had so much time isolated that it became that way. Perhaps it exists in such way it cannot be killed (ethereal for example).
It`s entirely possible for a peaceful alien race to exist which has not been touched by more powerful, violent races, if that peaceful race has some unique characteristic that protects it from violent conquest.
Remember, we must think out of the box. Too often we Humans think aliens should act like how we act. That`s not necessarilly the case because as I`ve said already, they`re alien, from alien worlds.
I'm more considering that life itself is a product of a biological evolution, and that follows some basic rules, such as the fight of survival over a limited amount of necessary resources. It doesn't really matter how the specific bodies look like, on what planets they thrive or what they are made of. But if a powerful lifeform exists then it needs to be explained how it happened that they rose to such great power - and that's usually via struggle.
But I know what you mean - some God or "Q" just set down for one evening and spawned a noncorporeal potent peaceful race to do good & watch over the biological ones. It's more fantasy than sci-fi, at least, such a process has never been observed anywhere. And its mission is also difficult, because pacifism doesn't work in a grande scheme were different playtheoretical strategies are battleing each other. So if "evil" race no1 proceeds to conquer a "good" race they'll have to choose sides. But who's to say that evil is really evil? Usually wars are fought over a conflict of interest - where both sides mutually view their opponent as "evil" (and themselves as the "good" ones)
Ironically our scientists constantly up the estimate on earthlike planets in our own galaxy, if my info is up to date it's currently around 10.000 such planets. And the natural occurence of pre-biotic molecules (that might give way to life if certain conditions are met) is more and more becoming a normality. The question is why haven't we been contacted by alien races, esp. since we have a history of 4bl years of life on earth (and that could be measured even from afar). Some scientist speculate that intelligent life has an inbred tendency to destroy itself once it gathers too much power via technology....
So let's discuss CLIMATE CHANGE!!!