How It Works: Dark Matter
By Anne Pawsey
There's been a lot of talk lately about dark matter, but astronomers have been a bit vague about what it actually is. It turns out there is a very good reason for this; they don't know. What they do know is that it's out there and very important for the current model of the universe to work.According to the current calculations only about 5% of the universe is made from normal matter, the stuff in stars, planets and us. The rest is made of something else which we can't see directly. This is dark matter.
Astronomers need this dark matter to account for how clusters of galaxies behave; many of these groups seem to hang together even though they don't have enough visible mass to keep them stable. Some other form of matter is needed to keep the galaxies together. Dark matter may also account for the density of the universe, as well as allowing physicists to work out all manner of complicated sums.
So we know it's out there, but what is it? Many different ideas have been thrown about, one being that it is made from neutrinos. These are tiny particles that appear in certain types of nuclear reactions, like those in the sun. Until recently they were thought to be massless but some very clever experiments have demonstrated that they actually do have mass (though not very much). However, there are a couple problems with neutrinos being dark matter. They move too fast and the mass that has been measured is far too small to account for the missing mass.
The current particle of choice is the WIMP (Weakly Interacting Massive Particle). Unfortunately, they are difficult to see as light goes straight through them, and they go straight through everything else. In fact, there are probably several thousand WIMPs going through you right now.All over the world, there are scientists sat down mines hoping to catch a glimpse of a WIMP. Why mines? Well, not just because it’s dark, but because they need to make sure that other pesky particles don't get to the detectors and cause false positives. So far no one has actually managed to catch one.
The alternative to the WIMP is the MACHO (astronomers will stretch a pun to its limit if you give them half a chance). These are massive objects that don't emit any light. Astronomers look for them by seeing how light is bent around them – a technique called gravitational lensing. They've found a few but not nearly enough to make all the sums work, so for the moment, WIMPs remain the preferred candidates.
The search continues…
More from Anne on her page.
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Image: Farley Samson
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