The search to find the “missing matter” in the universe has been unsuccessful so many times that some exotic suggestions are taken more seriously than they once were. As Sherlock Holmes famously said, “When you have eliminated the impossible, whatever remains, however, impossible, must be the truth.” In this case, there are many impossible ideas that are tested to see if they are impossible. One that has attracted enough attention that IFLScience was asked to discuss is Dyson Spheres. There are good reasons to conclude that these hypothetical spheres are not the issue you are looking for, but also to explore how we know this.
First, what is a Dyson sphere?
Only a small part of the Sun’s energy falls on its planets, while the rest escapes into space. In 1937, science fiction writer Olaf Stapledon wrote a book, Star Maker, which explored the ideas of far more advanced civilizations searching for energy. 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, eventually partially or completely surrounding the star.
Dyson noted that such structures would block visible light from the star to observers elsewhere, but radiate in the infrared. Consequently, he argued, one way to find advanced extraterrestrial civilizations might be to search for infrared-dominated spectra.
The idea captured the imagination of many and achieved a surge in popularity when the mystery of KIC 8462852 (also known as Boyajian’s star) emerged in 2015. KIC 8462852 undergoes significant dips in brightness at irregular intervals, a very large result for be the planets that block her light. There was so much speculation that the observed behavior could be caused by a partially formed dyson sphere that another nickname, the “Alien Megastructure Star”, became common.
What is Missing Mass?
There are actually two types of mass that our surveys of the local universe have failed to find. The most famous of these is dark matter, the mass needed to explain the motions of galaxies according to the laws of gravity. The other type of missing mass is more regular material, probably mostly composed 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 this category consists of regular elements because evidence from shortly after the birth of the universe allows us to calculate how much ordinary matter there must be in the universe today. When we look around we can only see about two-thirds of that amount.
There is much less missing mass in this category than dark matter, but still very scary. Among the explanations are large filaments of gas that stretch between galaxies
So can Dyson spheres account for some kind of missing mass?
Alas, almost certainly not.
Once people realized how cool Dyson Spheres would be and were entertained by the possible science fiction ideas of living inside something so large, physicists thought about practicalities. And it turns out that full Dyson spheres just don’t make sense.
The material for a Dyson sphere has to come from somewhere. It is highly unlikely that even the most advanced civilization would be able to remove matter from their star and turn it into something solid. If they could, they probably wouldn’t rely on stellar energy anyway. Therefore, the material of the sphere would have to be composed of planets, moons and asteroids.
Some star systems have more mass in orbit than ours, others perhaps less. But there’s no reason to think we’re exceptionally easy in that department.
This means that there wouldn’t be that much mass in the sphere itself, even if you used every scrap of solid material in the planetary system. If the question was meant to mean “Could the material in the Dyson Spheres be so large as to make up a large fraction of the missing matter?” then you need to explain where that issue came from in the first place. Scouring interstellar space and finding rogue planets or other sources of material so they can be turned into props for solar panels is unlikely to be practical.
Another way to interpret the question is, “Could there be billions of stars surrounded by Dyson spheres that trap all their light so we can’t see them, making the galaxy much more star-filled?” denser than we think?” This is generally what people mean.
The popular, but almost certainly incorrect, view of the Dyson sphere is one that builds steadily until the star is surrounded by a complete sphere.
However, given the amount of solid material in the Solar System, any completely encircling 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 large amounts of energy, making the whole idea a net waste.
If Dyson spheres exist at all, they are very incomplete, or thin “Dyson Rings,” or networks of patches that collect a few percent or less of the star’s light. These are sometimes referred to as Dyson Swarms.
If a star were orbited by a Dyson monstrosity, we’d see it, dimmed by the occasional collisions when the chunk got between us and it—the hypothetical situation that made KIC 8462852 famous. Dozens of stars have been identified where this might occur, although other explanations are more likely.
In a case like this, the star would not be absent for a long period. Consequently, our estimates of the number of stars in the galaxy would not be very wrong, if at all. A small small number may be responsible for only a small part of the missing case.
Even if a full Dyson sphere were to be built, a key feature of the concept is that it would radiate in the infrared. Dyson wanted us to be on the lookout for that kind of infrared signal. JWST and our other infrared telescopes cannot look everywhere, so they may have missed some such radiators. However, if these were common enough to solve the mystery of the missing mass, we should have seen them by now.