Quirks & Quarks

What evidence for dark matter has been found in the Milky Way?

The gravitational pull on stars and the speed with which they orbit in the Milky Way can only be explained by the presence of dark matter

Without the pull of gravity from dark matter the stars in our galaxy would fly away

An artist's impression of the Milky Way galaxy. The dark matter halo, shown in blue, is not actually visible. (ESO/L. Calcada)

This week's question comes from Terry Pitt-Brooke, from Salmon Arm, British Columbia. He asks:

What evidence for dark matter has been found in our galaxy?

Sean Tulin, a professor of physics at York University in Toronto explains that scientists are very confident that dark matter exists throughout the universe, but are always investigating new ways to understand what it is and how it's distributed. 

The evidence for the existence of dark matter in the Milky Way is strong. Scientists believe that our galaxy lives inside a huge cloud of dark matter called a halo. We cannot see dark matter directly, but it exerts a gravitational pull on the stars in the Milky Way. The stronger the pull of that dark matter, the faster the stars orbit in order to remain in their positions. 

By measuring velocities and positions of stars, it is clear that there is a lot of extra invisible mass in our galaxy exerting a huge gravitational pull. In fact about 90 percent of the mass of the Milky Way needs to be dark matter, or else stars would simply fly out of the galaxy. 

This dwarf galaxy is a satellite of our Milky Way. The motions of the galaxy's stars indicate that it is embedded in a massive halo of matter that cannot be seen (ESO/Digital Sky Survey 2)

The shape and precise distribution of the dark matter is a matter of debate. It could be spherical, squashed, stretched, smooth or clumpy. To understand more about dark matter and identify what precisely it is, Tulin says we need to see some effect beyond its gravitational pull. So far, although the search continues, no such effect has been found.