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How We See Dark Matter

Monday, February 2nd, 2009
Dan Coe
by Dan Coe
Astronomer

Planets, stars, buildings, cars, you and I, we are all made of the same basic stuff - atoms, the building blocks of matter. The late Carl Sagan famously said “we are star stuff,” as the heavy elements in our bodies were all forged in supernovas, the explosions of dying stars. In a real scientific sense, we are one with everything we see in the night sky.

We have since learned that everything we see is awash in another kind of matter, a “dark” matter, made of particles yet to be discovered. Dark matter is all around us, but we cannot see it. Some estimate that a billion dark matter particles whiz through your body every second, but you cannot feel them. We now believe that the universe contains five times more dark matter than ordinary matter. While we all may be made of star stuff, we find that the universe is mostly made of something very different.

Why do we believe that dark matter exists? How can we study something that we cannot see or even feel? And how can we unravel the universe’s greatest mystery - what is this dark matter?

The idea of dark matter was born at Caltech in 1933. (Just three years later, JPL would be born there as the “rocket boys” began their first launch experiments.) In observations of a nearby cluster of galaxies named the Coma cluster, Fritz Zwicky calculated that the collective mass of the galaxies was not nearly enough to hold them together in their orbits. He postulated that some other form of matter was present but undetected to account for this “missing mass.” Later, in the 1970’s and ’80’s, Vera Rubin similarly found that the arms of spiral galaxies should fly off their cores as they are orbiting much too quickly.

galaxy cluster
In this Hubble image, the galaxy cluster Abell 2218 reveals its dark matter by lensing background galaxies into giant arcs. Image credit: NASA/JPL.

Today dark matter is a widely accepted theory, which explains many of our observations. My colleagues and I at JPL are among those working to reveal and map out dark matter structures. Dark matter is invisible. But astronomers can “see” it in a way and you can too, if you know what to look for! For instance, if you have a wineglass on a table and you look through the glass, the images behind it are distorted. So too when we look through a dense clump of dark matter, we see distorted and even multiple images of galaxies more distant. Matter bends space according to Einstein’s Theory of General Relativity, and light follows these bends to produce the distorted images. By studying these “lensed” images, we can reconstruct the shape of the lens, or in our case, the amount and distribution of dark matter in our gravitational lens.

Our observations of dark matter in outer space force particle physicists to revise their theories to explain what we see. Hopefully through their efforts, physicists will soon produce dark matter in the lab, catch and identify a small fraction of that which passes through us, and ultimately explain the relationship between dark matter and “star stuff.”