In search of the missing link

Picture: ESO/HO

Melbourne scientists are hoping to unravel the mysteries of dark matter in a new laboratory to be constructed in the depths of a Victorian gold mine. But what exactly is dark matter? Tim Thwaites (BSc(Hons) 1974, Trinity College, Janet Clarke Hall) explains.

We all know what gravity is. It’s the force that keeps us grounded to Earth. It also keeps Earth tethered to the sun and the sun part of the Milky Way galaxy instead of careering off into space. Gravity is a property of mass or matter.

In the 1930s several astronomers contemplating the heavens noticed something was missing. In fact, a lot was missing. They recognised that all the matter they could see — the galaxies, stars, planets, dust clouds and comets — did not have enough gravitational pull to hold the universe together.

It took until the 1970s for the American astronomer Vera Rubin to accumulate enough solid evidence from studying how the galaxies move in space to suggest the existence of another form of matter that is invisible to us. And it isn’t just a small amount. It accounts for about 85 per cent of all matter in the universe. Because it does not interact with light, it was dubbed dark matter.

Since then the hunt has been on for particles of dark matter. Pretty well the only thing we know about them is that they must have gravity, so they can bump into and move things, and we can detect them through that action. The physicists proposing the underground laboratory at Stawell are hoping that dark matter particles will reveal themselves by causing nuclei in sodium iodide crystals to absorb energy and recoil.