My way of thinking about it is that a civilization capable of interstellar travel has enough energy and resources (which is probably the same) to terraform any “free” planet to their liking. For all we know, space is mostly devoid of any life. So you can build intergalactic empire for 100,000 years and still not encounter anyone. I see no point for such advanced species to conquer someone. The Dark Forest is an interesting concept but seems unlikely.