What evidence for dark matter has been found in the Milky Way?
Without the pull of gravity from dark matter the stars in our galaxy would fly away
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.
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.