Also we can use this as a trap. Just choose a star, make it darker and wait.
If there are other advanced societies out there I seriously doubt war is their main motivator in space exploration, the same way Voyager and Curiosity are unarmed. Sure, that makes for fun sci-fi stories, but real life is governed by fairly simplistic game theory rules, at least from a high-level perspective. The motivators we have been given through evolution will be the motivators they have as well. Unless we're dismissing evolution as only a local phenomenon, we should expect similar beings to ourselves. I think the 'hostile alien' scenario shouldn't be our default. Sadly, that approach sells ad impressions and schlocky futurist books and influences a lot of people to believe things that are pretty questionable.
If we discovered a primitive species in the European seas we'd be tripping over ourselves to preserve them and study them and to make sure they never, ever get hurt by us (or a stray asteroid or anything we could prevent). We wouldn't be conquering them with robot subs for their pretty coral jewellery or sub-surface oil or whatever. We have enough pretty things and energy here on Earth and in unihabited places if need be. Life is rare enough that we don't need to attack other life for resources, assuming we ever find any.
Arguably, if your species is mind-mindbogglingly stupid enough to declare war constantly and against the basic rules of game-theory then I doubt your society will survive the nuclear weapons stage anyway and won't ever be in the stars. The nice thing about the Drake equation is that it suggests that Klingon-esque societies don't make it to the space-faring stage, which is nice.
They are products of evolution and, in theory, we'll be very alike in many ways.
But then, maybe a species able to colonize a galaxy can only appear as consequence of those rare exceptions. We, as a not completely selfish species are evidence for that.
Humanity still very much depends on various resources today: fossil fuels, uranium, metals, fish stocks, etc. So wars over resources still very much exist. The sense of "duty to civilize the inferior races" is also alive and maybe even stronger than ever, though expressed with less offensive words.
I would expect a civilization sufficiently advanced to build a Dyson sphere to have reached some sort of sustainability with regard to resources. But what about ideology? What if they deem us barbarians that need to be civilized?
[1] I'm French, hence the "we".
[2] In Jules Ferry speech before the French Chamber of Deputies: http://sourcebooks.fordham.edu/halsall/mod/1884ferry.asp
Once you are post-scarcity, this kind of propaganda will be much less effective. It'll be more obvious its bullshit.
Probably not. There's around a billion stars in the Milky Way galaxy alone, and even more in nearby Andromeda. If some civilization has the technology to travel to our system (which is likely tens of light-years away if not much farther), then it'll also have the tech to go to many, many, many other systems other than ours, which also have habitable-zone planets, and are probably more conveniently-located.
The only real exception is if there's a race that's not all that advanced, and is located very, very close to us, such as in the Alpha Centauri system, 4ly away, so that we're just close enough to make travel here feasible for them, but other habitable-zone planets are significantly farther so that it makes more sense for them to attack us rather than just find someplace else.
But as we're already figuring out now, a mere 4ly is already a really, really long distance with our technology. Our longest-range probes have only gone a small fraction of that distance, and they've just about left the system. It'll be tens of thousands of years for them to go that distance at their current speed. So traveling that distance is already rather unimaginable for us without something like a generation ship or cryo-storage ("suspended animation"), and when you have that, why bother fighting a war if you can just continue on?
Finally, if you have the technology to build a generation ship that can work reliably for 10,000+ years supporting life, you probably have the technology to mine asteroids, build habitats on the Moon and Mars, build large habitats with artificial gravity (by spinning) in Lagrangian points, etc. In short, you already have the technology to make habitats perfectly suited for your species, and there's a whole solar system full of building materials at your disposal. If you have that, why would you want to travel such a long distance just so you can steal a planet from some other race, when it's very unlikely that planet will even be suitable for your biology without significant terraforming?
In a nutshell, the problem with this kind of thinking, and a lot of sci-fi in general, is that it assumes that interstellar space travel is much cheaper and easier than artificial habitat construction, when in fact the reverse is true. This is not to say that artificial habitat construction is that easy either; we still haven't perfected that for really long-term applications. But interstellar travel is really, really hard.
Now, of course, it is possible that we've completely overlooked some fundamental physics and it's actually not that hard to travel light-years very quickly; this would change the equation entirely. There's a Neal Stephenson short story about this, where some warlike and somewhat primitive aliens attack Earth, and are easily defeated by our far-superior weaponry (they basically had 19th-century cannon technology). So the humans get their technology and figure out that faster-than-light travel is actually really easy, we had just overlooked something simple all this time; at the end, the aliens are crying about what they've done, because then the humans set out to become galactic conquerors.
Is that maybe The Road Not Taken by Harry Turtledove?
I don't think there's ever going to be a point when a race has "enough" and doesn't want to explore or build any more. Our ambition should grow with our capabilities. Once you can control a star, why not work on a galaxy?
Every outpost of the empire will essentially have unlimited manufacturing capability and it will take decades to project force, decades the outpost can spend making sure the amount of matter they have devoted to war exceeds the tiny amount of war matter the central empire threw out into space.
The big idea of the book is that Einstein isn't wrong about C being the speed limit. Therefore, to be a starfaring race requires the ability to make commitments and hold to them over extremely long timespans. The connection to game theory is that, as in the Prisoner's Dilemma, the only way you can really make predictably right decisions in cooperation with other entities is if your decision-making formulas are spelled out mathematically in advance; so all can predict how you will act. The first book begins with mankind discovering the galaxy's "rules" on a kind of signpost at a nearby star.
Read the books!
My point is that the very concept of an interstellar resource war, and thus the concept of interstellar control, is incoherent given our present physics. You can take over a system that isn't technologically enabled, but that's it, the resources available to a technologically enabled system are greater than the resources that can be sent into the system.
So it's certainly feasible for an advanced race to build a star empire; the question is why they'd want to do it. If we're talking about humans you might say ambition or imagination. If Vulcans, maybe they'd think it logical to pre-emptively become stronger than any potential aggressors.
You could slam a bunch of mass into a system at extreme speeds, but then they just have to hop into life boats until the storm ends (if they have the capacity to ponder traveling the stars, they have the capacity to spend a decade on a space station).
For aliens. Just like we make traps for birds, so we can tag and study them.
https://en.wikipedia.org/wiki/The_Killing_Star
Warning: the "Plot Threads" section has spoilers.
That's a "current thinking" issue. Prior to industrialization, humanity (statistically per capita) didn't care about preservation. In search or resources you get marginalized conservation thinking. Never-ever is a fantasy, because over a long enough period of time every priority can and will change.