direct vs indirect dark-matter detection

If dark matter is made of particles streaming through us all the time, how could we ever catch one that almost never touches ordinary matter? Physicists pursue two complementary strategies, plus a third for completeness. Direct detection tries to feel the rare moment a dark-matter particle bumps into an ordinary atom. Indirect detection looks for the faint signals dark-matter particles might give off when they meet and destroy each other out in space. (The third route is to try to make dark matter in a collider; that belongs more to accelerator physics.)

Direct detection is like setting an extraordinarily quiet trap and waiting. Experiments place a tank of cold liquid xenon, or a crystal, deep underground — under a mountain or in a mine — to shield it from cosmic rays. They then wait for the tiny, rare jolt of a dark-matter particle nudging a single atomic nucleus, which would show up as a faint flash of light or a whisper of heat. The challenge is that ordinary radioactivity and stray particles mimic the same signal, so these are among the most painstakingly shielded, purest experiments ever built. Indirect detection, by contrast, watches the sky: if two dark-matter particles annihilate, they could produce a burst of gamma rays, antimatter particles, or neutrinos, and telescopes and neutrino observatories hunt for an excess of these coming from places where dark matter should pile up, such as the centre of the galaxy or the core of the Sun.

So far both approaches have produced limits rather than discoveries: ever-stricter ceilings on how strongly dark matter can interact, but no confirmed signal. This is real progress even without a find, because it rules out whole classes of theories. The fact that decades of searching have come up empty is itself shaping which dark-matter candidates physicists take most seriously, pushing attention toward lighter or more weakly interacting possibilities.