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Dark Matter Day: In search of the ghost of particle physics

The universe has a problem many people would envy: it’s really heavy, but doesn’t look it. Scientists from around the world are using research centres like Sudbury’s SNOLAB to find the answer: what exactly is dark matter?

Thousands of scientists are waiting for the bump in the night that would reveal the identity of dark matter. Nature is hiding in the shadows: roughly 85 per cent of the matter in the universe is dark matter, and we know almost nothing about it. 

Detecting dark matter has become a top priority for physicists seeking to understand the nature of matter. The quest to illuminate dark matter will be celebrated by scientists around the world on Oct. 31.

There are all sorts of questions sparked by this mysterious substance that fills our universe. Fundamentally, what is it? Once the experiments reveal an answer for that question, there will be follow-up experiments to understand how dark matter fits with what is known and how it can be used. How does dark matter interact with the world we already understand? How does dark matter interact with itself?

Scientists have been working to answer that fundamental question, what is dark matter, for decades. Early on, clumps of rocks and dust that were too cold to shine were the favourite explanation for the marked lack of matter in galaxies. However, that hypothesis was disproved and physicists today are exploring new theories. 

To investigate those theories, scientists from around the world are visiting Sudbury and building underground experiments that search for the rare signals of dark matter bumping into ultra-sensitive detectors. These detectors take many years to plan, fund and execute. 

The allure for those who have dedicated their lives to this science is that 85 per cent of the universe waits to be uncovered, and that understanding this mystery could fundamentally shift our view of nature.

Dark matter is the ghost of particle physics. The detectors set up to capture it have grown as scientists perfect their techniques. Some use 10 tonnes of sensitive material and planning is underway for 100-tonne scale detectors. Imagine building something the size of 50 rain barrels with a billion pebbles streaming through each second, and only handful of those pebbles ripple the surface of the water each year. 

This gives some idea of the ghostly nature of dark matter and the patience required of scientists building these experiments.

Waiting for that bump in the night leaves some time for self-reflection, and it can be sobering to contemplate that you could spend your life searching for an imaginary substance that doesn’t exist. 

Previous hypotheses for dark matter were wrong, and there are competing theories today (so at least one of those is wrong, too). However, dark matter is just too tantalizing a hint at an unseen universe for scientists to ignore. The evidence for the existence of dark matter has grown over the last century and cemented this mystery as one of the highest priorities in particle physics. Even to disprove one of the current hypotheses for dark matter would be a huge contribution to science as it would narrow down the search.

So, on Halloween, take a moment to think of the dozens of experiments testing theories of dark matter around the world. We have half a dozen world leading detectors right here in Sudbury, two kilometers underground at SNOLAB in Vale’s Creighton Mine. 

Scientists around the world are running other experiments at underground laboratories, particle accelerators, and with gamma-ray and x-ray telescopes. Legions of scientists at the cutting edge of technology, waiting for that bump in the night that will finally unveil the ghosts we call dark matter.

Dr. Jeter Hall is the director of research at SNOLAB.