The work was nearly complete. The junior officer watched as an almost unimaginably vast section of light-absorbent supersteel, completely black in every portion of the spectrum, was maneuvered into place. Or at least, he watched a diagrammatic overlay on the screen in front of him, since it is very hard to observe something that is completely black against the background of the rest of the universe, which at least in the visible spectrum is also almost completely black.
The piece in front of him was, according to the signals coordinated amongst the construction fleet, scheduled to be the very last plate inserted into the Dyson structure. Of course, the fact that it was the last was the result of sub-par construction work on the part of their team, which had delayed completion, and was therefore not to be celebrated. But still, it was fun to think that you'd be involved in the very final completion of an effort that had consumed your whole species' attention for several hundred generations. No ceremony was planned, but there was a lot of attention in the dataverse being paid to these final motions. The Open Universe movement was using the increased media attention to make some final grandstanding pleas for attention, but they were crackpots, a vanishingly small minority, and were not expected to make any attempt at actual physical protest.
A thought occured to him, as it had to many millions of others in his species.
"Why don't we send out a signal?"
His superior officer was standing next to him, also watching the final movements.
"Oh, you know all the reasons. It'd be too much effort for too little possible gain."
This argument had been played out for generations even prior to the beginning of the Dyson project. The junior officer knew this, as did his superior, but they had nothing better to do, and indeed following completion of this project the Dyson construction forces would probably be almost entirely disbanded apart from maintenance crews. They would turn their minds to other endeavours, and probably never see each other again. With this in the back of their minds, each indulged the other in retreading the familiar steps of the conversational dance.
"But think of it from the perspective of another civilization, just breaking out of the dark ages. They'd turn their telescopes to the sky and see... nothing. Or almost nothing, just a bunch of dead, uninhabited stars. Even after they worked out gravity and started making detailed calculations, there still wouldn't be anything to see, not in most of the electromagnetic spectrum. They'd have this weird idea of the universe, it would look like it was full of dark matter."
"And that's the point. It would be so obviously weird that they would spend a lot of time trying to find the dark matter."
"But what if they found the dark matter but didn't realize the significance?"
"Didn't realize that matter is never naturally dark, you mean?"
"Exactly. They might just think that's the way the universe works. Empty, dark. It would seem a terribly lonely, cold universe. They might get discouraged, and never try to break out of their solar system. Don't we have a duty to send out a visible signal for them, letting them know about faster than light communication, and how to build a Dyson sphere to power their civilization? At the very least, to let them know that there are other civilizations? That they're not alone?"
"Come now. Any intelligent civilization could look at their own world, their own solar system and see that it is rife with life. Anywhere that any form of life can possibly take root, there it is. Water-based biology on rocky worlds, gas-based life on the gas giants, crystal slowlife on ice worlds, intelligent plasma in the hearts of stars themselves... it's everywhere. They would immediately work out that the dark matter is dyson spheres, with civilizations inside of them."
"But then what?"
"What do you mean, then what?"
"What if the next step isn't obvious? What if they realized the universe is full of civilizations in dyson spheres but didn't discover faster-than-light communication?"
"Oh, you're just being deliberately obtuse now. Once you've seen that the universe is awash with countless hosts of completely dark, energy efficient dyson spheres, with absolutely no detectable attempts to communicate in any part of the electromagnetic spectrum, from visible light to radio waves to x-rays, it would just be inconceivably dense to assume that it was because nobody was interested in talking to each other. Apart from anything else, the sheer universality of the dyson sphere plan would make it obvious that some form of coordination is happening."
"Okay, so maybe they'd know that the electromagnetic spectrum isn't the way to go. But would they immediately work out the alternative?"
"Oh, it might take them a few false starts. But eventually gravity waves would occur to them. And once you start listening to them, well, it's impossible not to find intelligent life. Our biggest problem when we discovered them was sorting through all the overlapping signals to make sense of anything. That and building computers fast enough to follow what was going on in the galactic community. There was so much going on at once we thought there was a war."
"But what if they didn't? They could decide they were alone in the universe. They might think that all the way up to the heat-death of their star! When you think about it that way, isn't our lack of communication, denying them the ability to transmit their consciousness to the star factories, tantamount to genocide?"
"Look. If a species is arrogant enough to think it's the only intelligent life in the whole universe, and stupid enough to not go looking for anybody talking in anything but radio waves, do we really want to hear from them anyway?"
"I suppose you're right," said the junior officer, as the last rays of light to ever leave the dyson sphere made their escape.
Dyson spheres are a fun if not terribly realistic concept.