The following are excerpts from an interview in the Discover magazine special double issue on the science we don’t see. “Dark matter sounds like some physicist’s tall tale: There’s this invisible matter, see, and it has this powerful gravitational effect on galaxies. That’s why we know it exists. In fact, it outweighs ordinary matter by about five to one. Problem is, dark matter

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doesn’t reflect or absorb light, so we can’t see it. Oh, and it rarely interacts with conventional atoms, so we can’t feel it, either. However we know it makes up a huge part of the universe, so we keep looking for it. Physicist Elena Aprile is one of the leading lights in this dark business. She heads a prominent dark matter experiment called Xenon, which is based 5,000 feet underground in Italy’s Laboratori Nazionali del Gran Sasso, one of the world’s largest subterranean physics labs. Question: Seriously – what is dark matter

Aprile: The best answer is that we have no idea. We know dark matter is there. We’ve known it for more than 70 years. There was a 1933 paper by the Swiss astronomer Fritz Zwicky showing that visible matter is only a small fraction of the universe. Just 18% of the matter in the universe is composed of the stuff we know. The remaining 82% is what we call dark matter. Other discoveries in astronomy have since reinforced this view that something is missing. We know dark matter is there, but only from its gravitational effects. For example, the presence of dark matter helps explain why our galaxy is stable. The Milky Way is a disk that rotates like merry-go-round. The question is, what keeps it from flying apart? Gravity, of course, but there is not enough visible matter in the galaxy to account for the amount of gravity needed to hold it together. That’s why we know that there must be other matter there that we can’t see.

Question: What is it like to search for something that you may never find?

Aprile: It feels very exciting and almost like a duty. The fact that we don’t know if we will discover dark matter does not take way the necessity to try. The italian particle physicist Carlo Rubbia,, who was my doctoral thesis adviser at the University of Geneva, recently quoted Galileo at a conference on dark matter: Provando et riprovando – ‘Try and try again.’ This is the basis of experimental science. We must try and try again to find the truth. If we stop because their is no guarantee”