The most important currency for an interstellar civilization is astronomical data. Telescopes out past the ice moons of every star will gather images of the heavens, and relay that data to all of their neighbors. Together, triangulated, those observations act as a telescope as large as the space that they span — an eye light years across. Because each star will be recording vast amounts of data, and exchanging it with all of its neighbors, this interstellar exchange will constitute the primary market between systems.
No material is so precious to be worth shipping from star to star, unless it is a diplomat or relic — both comparatively low-volume exchanges. And other data will be exchanged, cultural and scientific, yet the rate of return on these diminishes rapidly. Only astronomical data will continue to grow, and be traded between every pair of stars continuously, at high volume. (Unless, of course, the US military was serious about their recent patent of an inertia-less Casimir shell…)
These observations serve various purposes — lidar pulses from networks of satellites will spot incoming debris, which is captured, because mass is precious energy; observations of surrounding star systems, when seen from many angles, provide details of our neighboring opportunities; vision of one’s neighbors will provide them with an early warning if a neighbor plans an attack — they can see the exhaust from the rocketing war machine.
Just because astronomical data becomes the largest interstellar market, I do not mean to imply that it would be a significant fraction of each star system’s economy. When we are done billowing gas away from the sun, by surrounding it with mirrors and magnets, we will have a core of rock and metal many times more massive than Earth, and an immense volume of fuel — the local economy will be staggering.
Yet, many will seek to launch ahead of everyone else, to distant stars, or even far off galaxies, in the hopes of founding their own empire, and growing immense by the time anyone from home ventured so far. To project the optimum distance at which to settle, and determine which cluster of galaxies is best, these trail-blazing would-be tyrants must not go so far away that an alien civilization has arisen where they might land. Distant galaxies take billions of years to reach, giving nature ample time to cook up another space-faring people. Heck, some galaxies may already be populated by galactic empires, appearing only as a dim red dust spread across a vast reach!
To plan their journey, they will need good astronomical data. And Earth will spawn a continuous stream of spacecraft, each figuring that they must settle a little bit closer to the Milky Way, so that they don’t bump into another rich psychopath. I call these the Edge Lords.
Others need that data for a slightly different purpose, though their disposition and goals are similarly narcissistic — to be a new Adam and Eve, king and queen of an entire galaxy. Maybe two. They recognize that their equipment can harvest valuable materials, and convert that material into more equipment most easily when they crunch through asteroids and moons. These small bodies are easier to prospect, mine, transport. You get a higher ROI than anyone else.
These, the Lunatics, will bounce like skipping stones, settling briefly in each star system they pass, to gobble debris and jet on. After hundreds of millions of years, the very first Lunatics will collide with the outskirts of the very last Edge Lord’s demesne. I don’t expect them to get along.
And, any low-tech aliens encountered by the Lunatics will be ignored, though they will be pelted by the iron debris sputtered from the Lunatic’s orbiting mines, and a home world moon’s absence will eliminate the tides, changing the biosphere radically. The Lunatics won’t have time to care. They are racing ahead of each other, hoping to swipe their competitors and double their own mass. They are jackals.
So, alien intelligence is most likely to first encounter an Edge Lord or Lunatic, who frantically fled the boundary of society in order to have absolute control over the incarnation of their perversions and wrath. We can expect the same from alien races: the first ones we meet will be the ones who are escaping their society. We need to keep watch. Stare unblinking at the void, in every place, hoping that the black stays black, that there is no sudden ignition of a craft beginning to slow, blazing its engines toward its destination. Astronomical data is priceless.