Searching for the missing Milky Way: Canada may find it

Our Milky Way galaxy has finally been weighed at an astounding mass of 1.5 trillion suns. But this creates a huge puzzle, because telescopes only see about 200 billion stars in the galaxy, which means the majority of the Milky Way is made of dark matter, and so far, no one knows what it is. But Canada is working on it.

The search for dark matter is happening in an unlikely place, 2 kilometres underground, near the city of Sudbury, Ont. SNOLAB, which was carved out of solid rock within a nickel mine, houses ultra-sensitive detectors that will hopefully capture dark matter particles, which means this country could provide the answer to what 85 per cent of the universe is actually made of.

Looking for dark matter in tiny flashes of light

One of the detectors, called DEAP-3600, is the largest of its kind in the world. The device uses a spherical vat containing 3600 kilograms of liquid argon, which the experimenters hope will give off tiny flashes of light when dark matter particles pass through it. Extremely sensitive light detectors surround the sphere will record the flashes and determine the direction in space the particles came from. This is similar to a technique that was used with a different liquid that led to the Nobel Prize-winning study of neutrinos in the same facility. 

Like neutrinos, which are ghostly particles that come from the sun and other stars, dark matter particles are believed to pass right through the Earth as though it is a clear pane of glass. That makes them extremely difficult to catch because they have a habit of passing right through any detector without stopping.

That's why SNOLAB is deep underground, so any other forms of radiation coming from space will be filtered out by the body of the Earth. The DEAP-3600 has been running for more than two years, and while it hasn't found any dark matter yet, the scientists increased its sensitivity by eliminating every other type of radiation that could be coming from the lab itself, such as natural emissions from the rocks of the Earth or even dirt on the floor. That makes SNOLAB one of the cleanest places on Earth, which is ironic, considering it is part of a working nickel mine.

Or maybe we'll find dark matter in a bubble

Not far away, in another part of the sprawling SNOLAB complex, another dark matter detector, the new PICO detector will use a bubble chamber filled with C3F8 liquified gas, which is expected to form tiny bubbles when dark matter particles pass through. One version of the PICO detector, the PICO-40L, is currently under construction and a larger version is in development. These detectors also require a super-clean environment, so all forms of radiation except dark matter can be ruled out.

Incidentally, when scientists use the term "dark," that is scientific terminology for "we don't know what it is." One candidate for the dark matter particles are referred to as WIMP, or Weakly Interacting Massive Particle. Weakly interacting means they are not affected by electric charge, or fields that hold atoms together. And since atoms are actually 99 per cent empty space, WIMPs usually pass right through those spaces or the spaces between atoms without being absorbed or bouncing off. It is only when they hit the tiny nucleus at the centre of an atom that you get an interaction. And since a nucleus is an unimaginably small target, a bull's-eye hit is a rare event.   

WIMPs are called massive, even though they are subatomic in scale, because when assembled in large quantities, their mass produces the gravity that holds entire galaxies together. In fact that's how dark matter was first proposed. Galaxies such as our Milky Way are spinning discs with gravity pulling towards the centre, preventing the stars from flying off in all directions like water from a lawn sprinkler. But if you add up the mass of all the stars in the galaxy, they don't produce enough gravity to do the job. Not by a long shot. That means there is a huge invisible mass of invisible matter among the stars forming a huge halo around the galaxy that is providing that extra gravitational force.    

Finding dark matter is discovering the very fabric of the universe. When you look up on a clear dark night, the shimmering stars you see are only a tiny part of what's really out there. The blackness is what the universe is actually made of.

Most Canadians are unaware of the world class work being carried out beneath the city of Sudbury, but if dark matter is detected there, it will place that town and Canada on the world map.