An international team of scientists, including several from the University of Toronto, have generated a groundbreaking map of dark matter splayed across a quarter of the sky — and it confirms Albert Einstein’s predictions from more than a century ago.

The results, published Tuesday morning across three preprints in Cornell University’s open-access journal arXiv, are the fruits of 15 years of labour by over 160 collaborators at Chile’s Atacama Cosmology Telescope.

The manuscripts have since been submitted to the Astrophysical Journal.

“It’s extremely important for us to try and unravel many things about dark matter, how it’s distributed throughout the universe,” said Richard Bond, a study author and professor with the Canadian Institute for Theoretical Astrophysics at the University of Toronto.

“We have very good images of it, but we still don’t know exactly what it is.”

What is dark matter?

Dark matter is hypothesized to compose over 85 per cent of matter in the universe. Yet its very existence used to be the topic of fierce debate. The substance is completely invisible, not emitting or interacting with light or other electromagnetic radiation. Ordinary matter has only a sixth of dark matter’s density, Bond said.

Despite its invisible nature, dark matter is suspected to play a critical role in the development of the universe: “Dark matter is what basically holds galaxies together into clusters of galaxies … it’s dark matter which defines the basic structures that the stars form in,” Bond continued.

The stuff isn’t spread evenly across the universe, instead clumping together into lumps that later become the home of stars and galaxies. Studies have shown galaxy clusters would tear themselves apart if they only contained the visible mass detected by conventional measurements.

How can we map dark matter?

While we’re unable to see dark matter, we know it has mass and therefore a gravitational pull. As Einstein theorized over a century ago, light is bent or distorted by the gravity of massive objects — an effect called “gravitational lensing.”

Therefore, by measuring the amount that light is warped by gravity in its journey through the universe, scientists can get a clear picture of how dark matter is distributed.

“We used the oldest light in the universe, called the cosmic microwave background, to make a measurement of how matter is distributed on a larger scale in the universe,” said Adam Hincks, a study author and assistant professor at U of T’s David A. Dunlap Department of Astronomy and Astrophysics.

The cosmic microwave background, sometimes called the “baby picture of the universe,” is the light that emanated shortly following the birth of the universe — known as the Big Bang. Although invisible to the naked eye, scientists can track slight fluctuations in the light’s 18-billion-year-long voyage across the universe using sensitive instruments.

“The reason this is exciting is, well, first of all, we’re making that map,” Hincks said. “But the map that we made, its properties really matched up exactly with what we expected.”

What do the results mean?

“By looking at the oldest light, we can see how matter was distributed very early in the universe, how lumpy it was very early in the universe,” Hincks said.

Scientists can then take that map of the infant universe and, through Einstein’s theory of general relativity, calculate how the distribution of matter might change over billions of years.

“And then when we look at it, billions of years later, it’s grown exactly in the way that we expected,” Hincks continued. “So it’s a really, really nice piece of empirical confirmation that we understand how the universe grows and evolves.”

Dark matter has been mapped before — Hincks’ team was actually the first to measure it via gravitational lensing back in 2011. At the time, however, their map was “fuzzy,” making it difficult to analyze details.

Although their new map isn’t the largest of dark matter ever captured, it’s probably the most detailed, Bond said. “Before, we were limited in terms of the angular resolution of looking at these dark matter blobs on the sky. Now, we’re looking at it with much higher resolution, and therefore we’re getting more information about smaller and smaller scales.”

After operating for 15 years, the Atacama Cosmology Telescope was decommissioned in September, 2022. Scientists are now parsing through its data, which included generating this dark matter map. Bond said they’ve got additional results coming.

“The cosmological story leading to the formation of galaxies and the formation ultimately of planets is an amazing story of our time — and it’s amplified quite a bit by this new data,” Bond said.

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