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Revealing the Dark Side of the Cosmos

by Nick Kaiser

A burning issue in modern astronomy is the dark matter problem: Most of the matter in the Universe is in some mysterious invisible form. We can detect its presence, but we are still quite unsure of how much of this stuff there is, and we are totally in the dark (so to speak) as to what it is made of.

Dark matter is ubiquitous. Galaxies-like the Milky Way in which we live-are surrounded by halos of dark matter. The total mass in these halos is at least ten times that of the ten billion or so stars that a typical galaxy contains.

On the largest scales, galaxies seem to be distributed uniformly across the cosmos. On smaller scales, they tend to clump together. Giant clusters, which contain about one thousand galaxies, are the largest gravitationally bound structures in the Universe, and, it turns out, contain even more dark matter.

Contour lines of the mass of  a bright X-ray galaxy cluster as determined by gravitational lensing are plotted over an   image of the cluster, MACS J2243. Gravitational lensing is a technique astronomers use to measure dark matter.

Astronomers describe the amount of dark matter in terms of the mass-to-light ratio, and use the units of solar mass and solar luminosity, that is, the amount of matter in the Sun and the amount of light the Sun generates. In a galaxy, for every unit of solar luminosity, there is about thirty solar masses of dark matter. For galaxy clusters, this number is more like three hundred! That is a whole lot of dark matter.

If we can't see this stuff, how do we know it is really there? The answer is that we sense its presence from its gravity. For example, we can tell how massive the Sun is from the gravity required to hold the planets in their orbits; without this gravity, the planets would fly off into interstellar space. Similarly, in giant clusters we see the galaxies whizzing around at speeds of about 620 miles per second, and we can deduce how much mass must be present to stop the clusters from flying apart. For galaxies, the presence of their dark matter halos was revealed from measurements of clouds of gas orbiting around them.

The evidence that there is a whole lot more to the Universe than meets the eye is compelling, but there is still a lot we don't know. Some outstanding questions are, How is the dark matter distributed? How much is there? What is this stuff made of?

While we have partial answers to the first two questions, we are still a long way from having a complete inventory of the dark matter. We know there is dark matter inside clusters, but what about outside? There, we are much less sure. As to what it is made of, we are even more uncertain. There is no shortage of speculation, and theorists have explored all sorts of possibilities, ranging from elementary particles ten thousand times lighter than an electron to black holes weighing up to about one million solar masses.

Recently, scientists have developed a new technique that can help answer the questions about the distribution and quantity of dark matter. The technique is called gravitational lensing (see figure lower left). Just as the gravity of the Sun bends the paths of planets into circular orbits, very massive objects such as galaxies and clusters of galaxies actually deflect the light coming to us from more distant objects. Massive clusters bend the light so much that we can sometimes see multiple images of background galaxies. This only happens when the light rays pass very close to the center of the cluster, where the density of matter is very great.

But there are measurable effects even for galaxies that do not lie directly behind the cluster center because naturally occurring gravitational lenses- unlike the carefully designed lens in a camera or telescope-are far from perfect. Imperfect lenses distort the shapes of objects, just like carnival mirrors distort objects. This is the effect that is exploited in a technique called weak lensing. Here the objects in question are distant galaxies. These measurements allow the  reconstruction of the spatial distribution of the dark matter in the lens, giving us a picture of the dark matter in and around clusters.