Twenty millikelvins. That is two-hundredths of a degree above absolute zero, about 100 times colder than the void between galaxies, and it is the temperature now holding steady inside a nickel mine two kilometres beneath Sudbury, Ontario.

The Super Cryogenic Dark Matter Search — SuperCDMS — announced on March 17 that its detector array at SNOLAB has reached base operating temperature after years of construction and a months-long cooldown. The milestone marks the experiment’s transition from building to hunting.

What it is hunting: dark matter, the invisible substance that accounts for roughly 85% of all matter in the universe. We know it is there because galaxies rotate faster than their visible mass should allow. Beyond that, it has refused every attempt at direct detection.

SuperCDMS takes a different approach from its predecessors by targeting lighter particles — those weighing roughly half to five times the mass of a single proton. Its 24 detectors, hockey-puck-sized crystals of ultra-pure silicon and germanium, are fitted with superconducting sensors that only function at these extreme temperatures. When a dark matter particle strikes a crystal, it produces a faint vibration and a tiny electrical signal. The cold eliminates thermal noise from jittering atoms, letting those whisper-quiet signals through.

“For a one-of-a-kind system, it’s gone remarkably smoothly,” said SLAC scientist Noah Kurinsky.

The collaboration — over 100 researchers across 25 institutions — expects to begin collecting science-quality data by mid-2026. They do not expect many hits. According to Fermilab scientist Lauren Hsu, the interaction rate is so low that “we don’t expect to see more than a few events per year.”

A few events per year, pulled from the silence of nearly absolute zero, two kilometres from daylight. That would be enough.

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