From theory to experiment — the search for dark matter

The Nobel Prize in physics was awarded this week, and Canadian-born Dr. James Peebles shared the prize for his groundbreaking theories about the evolution of the universe. One of his audacious theoretical insights was the prediction that dark matter plays a huge role in the cosmos. Understanding the nature of this mysterious matter is a huge challenge and the leading edge of that work is happening in Canada. 

Considerable observational evidence has led physicists to believe that dark matter exists throughout the universe. We can see the influence of enormous clouds of it surrounding galaxies — with the stars embedded in it. In fact, it's believed to have been fundamental in the process that formed galaxies like our own Milky Way during the time when the universe was very young.

Astronomers know it exists because it has enough mass to produce the gravitational pull that hold clusters of galaxies together and can even bend light through an effect called gravitational lensing. 

But we've never seen it directly. Though it's called dark matter, it's actually not dark, but invisible. It doesn't show up in telescopes, and apart from its gravitational pull, it doesn't interact with normal matter in any way. We know it's there, but we really don't know what it is. The best guess is that it's some kind of intangible particle with mass but no other known properties

That is a bit frustrating for scientists. Until someone actually finds a way to detect dark matter directly, it will remain one of the biggest mysteries in modern cosmology because it makes up so much of the universe. As near as we can tell, there's about five times as much dark matter in the universe as regular matter. In other words, the proportion of the universe we can see, the stuff the stars, planets and people are made of, is a small fraction compared to the proportion that's invisible and intangible

So how do you hunt for something when you don't know what you're looking for?

Deep underground, two kilometres below the surface, in an active nickel mine near Sudbury, Ont., is the world's deepest and cleanest laboratory, known as SNOLAB.

This world-class facility solved the mystery of a different set of ethereal particles known as neutrinos, which led to a Nobel Prize for the lab's former director Dr. Art MacDonald in 2015. 

Now SNOLAB has turned most of its underground experiments over to the search for dark matter using a variety of exotic and super-sensitive detectors that scientists hope will capture the elusive dark matter particles.

Theorists suggest that these particles don't often interact with ordinary matter, which means they can pass right through the Earth as if it wasn't there. However on vanishingly rare occasions a dark matter particle might hit the nucleus of a normal atom just right, and that event could be detected. That's why SNOLAB is two kilometres underground. That much rock filters out most cosmic particles while dark matter passes through.

SNOLAB has several detectors running or under construction. 

• DEAP-3600 involves a vessel holding 3600 kg of liquid argon at -186C that is supposed to give off flashes of ultraviolet light when dark matter particles interact.
• PICO uses bubble chamber technology with a superheated fluid that creates a bubble when it is hit. 
• DAMIC uses the silicon of charge-coupled devices like those that are used in digital cameras. 
• Super CDMS uses silica and germanium crystals to detect dark matter. When a dark matter particle hits the crystals, it deposits a small amount of energy. The crystals must be kept at extremely low temperatures to reduce thermal noise and must operate at nearly absolute zero (15 miliKelvin).
• NEWS-G uses spherical proportional counters to search for dark matter particle interactions. 

All of these experiments are international co-operations involving dozens of scientists from many different countries all focusing their work in Sudbury. That means there is a good chance that the mystery of dark matter could be solved in Canada. 

So why so much effort to discover dark matter, something we can't even see?

It likely won't affect our daily lives at all. This is fundamental science asking basic questions about the nature of the universe — in this case, what most of the universe is actually made of. That's because dark matter, along with dark energy, an equally mysterious phenomenon, together make up 95 per cent of the universe. 

What we actually see in our telescopes, and what we ourselves are made of is only a small fraction of what is really out there. And we haven't got a clue to what all that dark stuff is.

It is amazing to think about the fact that here, in the 21st. century, with all our modern technology, we still don't know what most of the universe is made of. 

Understanding the fundamentals of who we are and how we got here has historically led to the greatest discoveries in science. In physics we learned about the forces of electricity and magnetism, the power within the atom, in chemistry we discovered miracle products, many of them making up the clothes on our backs. Advances in medicine, biology, ecology and geology have all come from scientists asking fundamental questions.

Who knows what the discovery of dark matter will bring? Perhaps we will have dark matter beams that will allow us to communicate directly through the Earth. Maybe we won't find an application for it. Even so, it's a question worth asking and an experiment worth trying if for no other reason than to gain knowledge just for the sake of knowing it.