The search for the “missing matter” of space has failed so many times that some exotic proposals are taken more seriously than they once might have been. As Sherlock Holmes famously said, “When you have removed the impossible, whatever is left but improbable must be true. In this case, many improbable ideas are tested to see if they are impossible. One that has attracted enough attention to be asked about by IFLScience is Dyson Spheres. There are good reasons to conclude that these hypothetical spheres are not what you are looking for, but also to examine how we know this.
First, what is a Dyson Sphere?
Only a tiny fraction of the sun’s energy hits its planets, the rest escapes into space. In 1937, science fiction writer Olaf Stapledon wrote a book, Star Maker, which explored the energy-seeking ideas of much more advanced civilizations. The book inspired physicist Freeman Dyson to propose that such civilizations could build giant thin surfaces in space to capture more of their stars’ energy and eventually partially or completely surround the star.
Dyson noted that such structures would block visible light from the star to observers elsewhere, but would emit infrared radiation. Consequently, he argued, the way to find advanced extraterrestrial civilizations may be to search for infrared-dominated spectra.
This idea caught the attention of many people and achieved a surge in popularity when the mystery of KIC 8462852 (also known as Boyajian’s Star) emerged in 2015. planets blocking its light. There has been so much speculation that the observed behavior may be due to a partially constructed Dyson sphere that another nickname, the “Megastructure Alien Star”, has become common.
What is the missing mass?
There are actually two kinds of matter that our surveys of the local universe have failed to find. The more famous of these is dark matter, the matter needed to explain the motions of galaxies according to the laws of gravity. The second kind of missing matter is more regular material, probably composed mostly of hydrogen and helium, as opposed to dark matter, which is most likely exotic particles.
When astronomers talk about “missing mass,” they mean the second kind. We know that this category is made up of regular elements because evidence from shortly after the birth of the universe allows us to calculate how much ordinary matter there should be in the universe today. When we look around, we only see about two-thirds of that amount.
Much less matter is missing from this category than dark matter, but there is still an awful lot of it. Explanations include giant filaments of gas stretching between galaxies
So could Dyson Spheres account for some kind of missing matter?
Unfortunately, almost certainly not.
Once people realized how cool Dyson Spheres would be, and entertained the potential sci-fi ideas of living inside something so amazingly huge, physicists considered the practicalities. And it turns out that complete Dyson Spheres just don’t make sense.
The material for the Dyson Sphere would have to come from somewhere. It is highly unlikely that even the most advanced civilization could take matter from its star and turn it into something solid. If they could, they probably wouldn’t rely on star power anyway. The Sphere material would therefore have to be made of planets, moons and asteroids.
Some star systems have more mass in orbit than ours, others probably less. But there’s no reason to think we’re unusually easy in that department.
That means there wouldn’t be that much mass in the sphere itself even if you used every bit of solid material in the planetary system. If the question was intended to be “Could the material in the Dyson Spheres be so huge that it makes up much of the missing mass?” then you would have to explain where the matter came from. It is unlikely that it would be practical to scour interstellar space for rogue planets or other sources of material to use as substrates for solar panels.
Another way to interpret this question is: “Could there be billions of stars surrounded by Dyson spheres that trap all their light so we don’t see them, making the galaxy much more densely packed with stars than we think? That’s what people generally mean.
A popular but almost certainly incorrect vision of a Dyson sphere is one that keeps building until the star is surrounded by a complete sphere.
However, given the amount of solid material in the Solar System, any completely enclosing sphere would have to be very thin. So thin, in fact, that it would be gravitationally unstable. The only way to avoid disaster would be to use massive amounts of energy, making the whole idea a net loss.
If Dyson spheres exist at all, they are very incomplete, either thin “Dyson rings” or networks of fields collecting a few percent or less of the star’s light. These are sometimes referred to as Dyson Swarms.
If the star were orbiting the Dyson Swarm, we’d see it, darkened by the occasional wobble, as that part got between us and it—the hypothetical situation that made KIC 8462852 famous. Dozens of stars have been identified where this could happen, though others are more likely explanation.
In that case, the star would not disappear for a long time. As a result, our estimates of the number of stars in the galaxy would not be too bad, if at all. Any small undercount could only be responsible for a tiny fraction of the missing mass.
Even if a complete Dyson Sphere was built, the basic feature of the concept is that it will emit infrared radiation. Dyson wanted us to look for this kind of infrared signal. JWST and several of our other infrared telescopes can’t look everywhere, so they may have missed a few such emitters. However, if they were common enough to solve the mystery of the missing mass, we should already be seeing them.