What makes up 96% of the universe? Not rocks or dust, or even microscopic aliens. No, the universe is mostly dark. And that shouldn’t elicit a “well done Sherlock Holmes – I can see that most nights when I’m having an essay crisis in the library at three in the morning” sort of reply. When we say it’s 96% darkness, we mean it’s made up mostly of dark matter and dark energy. This is, however, another way of saying that we have just about no idea what it’s made of, because dark matter and energy are names that we have given to substances that we don’t understand. We didn’t even have much evidence – until now.
Gamma rays originating from the heart of the Milky Way have been detected by a research group using the Fermi Gamma-ray Space Telescope. At first scientists were skeptical, but the most recent analysis of the results, carried out by a team including Daniel Hooper, from the university of Chicago, has revealed that the signals are most likely due to a particle which is thought to be hiding under the shroud of dark matter – known as the WIMP. WIMP is not just an abusive name from a rather irate astronomer – it stands for Weakly Interacting Massive Particle. When these WIMPs annihilate one another, they are predicted to emit electromagnetic radiation in the gamma region, exceedingly similar to what has been observed. The team in their paper confirmed that the analysis of the results were “in good agreement with that predicted by simple annihilating dark matter models”, implying that there are now few other alternatives to the dark matter theory.
The reason for the lack of direct evidence for dark matter is that it does not interact at all with electromagnetic radiation, i.e. visible light and the whole spectrum of waves above and below. Right, you say, but even when it’s completely dark, I can just feel my way around – I may trip over a few textbooks but I find the way to the bathroom eventually. Well, dark matter does not experience a nuclear strong force either, so not only can we not see it, it will not be held in any atoms. We can’t see it, touch it, or use any other sensory means to establish its existence. We must, therefore, rely on more indirect evidence, in this case the annihilation of WIMP particles, to infer their existence.
One force dark matter is predicted to experience is gravity. If it were found, its presence could help to explain the continued expansion of the universe, and also the behaviour of various galaxies that is not explained by the visible matter inside them. This was one of the lines of reasoning that lead to the dark matter hypothesis in the first place. Now, we may be well on the way to confirming this theory and explaining one of the great unsolved mysteries of the universe.
However, this is not the discovery of a new field of substance in our universe yet. In order to confirm the theory, research must be done into other systems which are predicted to exhibit similar behaviour, beyond our Milky Way, such as certain dwarf galaxies which orbit around us. Should similar signals be seen, this would provide further evidence for the theory of dark matter and help to confirm it. Only then will we be able to say with confidence that we are taking our first look into the darkness of the universe, and seeing what is really there…
PHOTO/ NASA/Goddard Space Flight Centre Scientific Visualisation Studio
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