The Mystery of the Missing Mass is not, as you might first suppose, the title of an Agatha Christie novel about a church service that failed to occur on schedule. It is, rather, one of the hottest (or coldest, depending on which theory you subscribe to– never mind, I’ll explain later) issues in the study of the universe.
Current theory holds that the universe was born in a Big Bang, a great explosion, and has been expanding ever since. Since this view became generally accepted, scientists have been attempting to figure out if the universe is “open” or “closed”; that is, will it continue expanding until it thins out into next-to-nothingness in the unimaginably far future, or will it at some point cease expanding and begin to fall back in on itself as gravity overcomes its outward momentum?
The answer to this question depends on the mean density of all the matter in the universe. If that mean density is below a critical level, the universe will continue expanding. If the density is above that level, the universe will eventually collapse.
One way to estimate that density is to add up the masses of all the observable galaxies and divide the total mass by the volume of space which has been investigated. The result of this kind of survey is not encouraging; there’s only enough visible matter to bring the mean density to one or two percent of the critical level.
However, there is good evidence that there’s a whole lot of some kind of “dark” matter (so-called because we can’t see it) out there, too. For example, galaxies rotate so fast that if there were not a lot more matter in them than just their visible stars, they would fly apart. As well, the galaxies themselves form giant clusters and other structures that we wouldn’t expect if there were not not more matter at work than we can see.
It’s possible that there is a lot more ordinary matter we’re just not seeing–stars called “brown dwarfs” that don’t radiate enough light for us to observe them, Jupiter-sized planets orbiting almost every star and vast clouds of gas and dust–but so much ordinary matter would be required to create the visible effects that this possibility seems unlikely. As one scientist put it, “a conspiracy of nature in dynamically dissimilar systems of almost all sizes” would be required.
Of course, “unlikely” is in the eye of the beholder. Cosmologists prefer to focus on other forms of matter that, to the non-scientist, seem even more unlikely. Neutrinos, for example. These are non-charged sub-atomic particles that are generally considered to have no mass. (A rather hard concept to grasp in itself.) But it’s possible that in fact they do have a slight mass. If so, they are such common particles that they could account for a great deal of the missing mass.
Because theories require neutrinos to be moving at extremely high speeds, they’re considered a form of “hot” dark matter. Trouble is, because they’re moving so fast, they couldn’t form galaxy-sized clumps, which is a problem since dark matter is needed to explain galaxy formation, too.
Another (rather weird) possibility is that massive string-like objects called, somewhat unimaginatively, cosmic strings, formed from the fluctuating energy fields of the early universe, could provide the mass.
However, the favored model is (or has been) the existence of “cold” dark matter; cold because, unlike neutrinos, the particles that make it up would be moving very slowly (and could therefore clump as required). Scientists have proposed many different kinds of cold dark matter, involving exotic elementary particles like axions, photinos, gravitinos, heavy magnetic monopoles, etc., all of which have some theoretical basis–and none of which have ever been detected.
But the fact that the theory requires hypothetical and unconfirmed particles is not a problem. Such particles would be extremely difficult to detect, although efforts are constantly underway. Trouble is, even if found, they may not solve the mystery of the missing mass, because now doubt is being cast on the whole cold dark matter theory.
An article in a recent edition of Nature by Will Saunders of the University of Oxford and associates uses a recent survey of the sky by the Infrared Astronomy Satellite to show that large-scale structures in the universe–gatherings of galaxies–occur too frequently to mesh with predictions based on the cold dark matter theory.
So where does that leave dark matter, and the nature of the missing mass? Up in the air (so to speak) at the moment. It may prove that a number of different kinds of dark matter are required to explain the structure of the universe. But cosmologists won’t be happy about that; it smacks a bit too much of that “conspiracy of nature” referred to before. They’d like everything to tie up nice and neatly.
And whatever the nature of the missing mass, there’s still the question of whether or not there’s enough of it to make the universe closed or open. All the calculations thus far, even when dark matter (whatever it may be) is factored in, have failed to resolve that question–the answer keeps coming out annoyingly close to the critical level; like a tennis ball right on the line, it’s too close to call one way or the other.
While I have no personal preference as to whether the universe is open or closed, I do confess to a strange attraction to the notion of cold dark matter. As a science fiction reader I love the idea of a universe filled with bizarre particles of a completely different kind of matter than we are used to. I’m sure Mr. Spock would approve.
Who knows? It may yet prove to be true, as was thought in the days before Galileo, that the stuff of the heavens is not the stuff of Earth.