Dark Matter: 'What Is Essential Is Invisible'

A photo of the galaxy Abell 1689. i

Abell 1689 is one of the most massive galaxy clusters known. The gravity of its trillion stars, plus dark matter acts like a two-million-light-year-wide "lens" in space. N. Benitez/NASA hide caption

toggle caption N. Benitez/NASA
A photo of the galaxy Abell 1689.

Abell 1689 is one of the most massive galaxy clusters known. The gravity of its trillion stars, plus dark matter acts like a two-million-light-year-wide "lens" in space.

N. Benitez/NASA

"What is essential is invisible to the eye," said the fox to the prince in Saint-Exupery's classic fable The Little Prince. And this is not only true when we refer to human emotions; in the universe, too, what is essential is invisible to the eye.

During the 20th century, astronomers collected a huge amount of information about the distribution of galaxies in the sky, their properties and composition. Even as early as the 1930s, Fritz Zwicky from Cal Tech noticed that whenever galaxies collect in clusters, they fly about each other at surprisingly high speeds.

His conclusion? A lot of invisible matter surrounds these galaxies, like ethereal cloaks, and that it is the gravitational pull from all this extra matter that causes the surprising motions of galaxies about each other.

Now, eight decades later, the case for the existence of dark matter, that is, matter that is around galaxies but doesn't shine, is very clear cut.

That there is dark matter shouldn't be a surprise. Even if some of us are real bright, we are all dark matter, because we don't produce our own light. At most, we emit radiation in the infrared, invisible to the human eye. Same with planets and their moons that also only reflect visible light.

But what is surprising is that this dark matter hovering about galaxies is something quite different: it has nothing to do with ordinary matter, matter made of protons, neutrons and electrons. Problem is, we have no clue what it is.

Well, we do know a few things. For example, it seems that most of dark matter is made of tiny particles that interact only very weakly with ordinary matter. Sometimes they are called "wimps" (weakly interacting massive particles.) There are some suggestions to what these wimps could be, but so far we are not sure.

Recently, results were announced on a search for wimps in an underground mine in Minnesota, the CDMS experiment. In spite of much expectation, the results are still uncertain. The search goes on. Meanwhile, dark matter has inspired some wonderful books, such as Philip Pullman's His Dark Materials trilogy.

We also know that dark matter is about six times more abundant than ordinary matter. So, our material stuff is clearly subdominant when it comes to galactic and cosmological dynamics. We have just heard the results from UCLA astronomer David Law and collaborators that the shape of the dark matter cloak surrounding our own Milky Way is similar to that of a squashed beach ball. Again, a big surprise; it would make much more sense if it were simply a nicely spherical blob surrounding our galaxy, as nearly symmetrical as possible. This is because, as stars, large mass objects that form due to gravity tend to assume spherical shapes. Why then does our galaxy's dark matter cloak have this funny shape? What about other galaxies? We still don't know. But the message is clear: Nature really doesn't care much for our expectations of symmetry and order.



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