Cold Dark Matter

To catch the whisper of a passing particle of dark matter, two experiments in northern Minnesota get cold. One of them, in fact, is chilled to a few thousandths of a degree above absolute zero.
Dark matter appears to make up about five-sixths of all the matter in the universe. Astronomers see it on large scales because it exerts a strong gravitational pull on the normal matter around it.
Yet no one has ever seen an individual bit of dark matter. It rarely interacts with normal matter in any way — and it may not interact at all.
The experiments in Minnesota use detectors made of germanium. If a particle of dark matter rams into the nucleus of a germanium atom, the nucleus should “wiggle” a bit, producing a tiny amount of heat and a tiny change in electric charge.
But the detectors need an enormous amount of shielding. They’re in a laboratory that’s a half-mile below ground; the layers of rock screen out cosmic rays, which can zap the germanium atoms.
At normal temperatures, the atoms vi