The Mystery of Dark Matter

Despite the doom and gloom of approaching deadlines, a sizeable group of people joined us as we virtually joined Katie Mack who gave a wonderful talk about dark matter and its mystery in the universe. Katie Mack is a theoretical astrophysicist, currently holding the position of Hawking Chair in Cosmology and Science Communication at the Perimeter Institute for Theoretical Physics. Her research includes dark matter and vacuum decay. She is the author of the book "The End of Everything (Astrophysically Speaking)".

The talk was incredibly easy to follow - thank you Katie for making it accessible to non-astrophysicists!

Everyone was captivated by the comparison of Dark Matter to The Force!

A Short Summary

What is dark matter? Dark matter is an invisible energy, which does not interact with light. Since mass and energy can be converted (E = mc²) and dark matter has mass, we know it is also energy. It provides extra gravity that holds the galaxies together in space. The fascinating thing is that it can pass through itself and other matter.

Intracluster light (blue) in the galaxy cluster Abell S1063. One technique uses intracluster light, imaged by Hubble, to map and study dark matter. (Credit: NASA, ESA, and M. Montes. University of New South Wales, Sydney, Australia)

She went on further to explain that we know it exists because we can see how stars and galaxies rotate and where the visible mass is (the visible mass is usually in the centre). The centre, therefore, has more gravity. Stars would typically rotate faster with more gravity. But, stars in the outer galaxy actually move at the same speed - implying there is extra matter keeping them going around without drifting and flying off into space. So, it is not just the gravity of the visible matter that holds the galaxies together. Incredible stuff!

Einstein Ring Galaxy SPT-S J041839-4751.8. These can help us observe dark matter (JWST image, u/Spaceguy44)

Dark matter can be observed through gravitational lensing. This phenomenon shows us just how much matter there is, both visible particles and invisible dark matter, as it passes infront of other galaxies. So we know it is there, we just have not found its particle yet. Dark matter is made up of particles beyond our standard known model of particle astrophysics. This hypothetical type of particle is commonly referred to as Weakly Interacting Massive Particle which interacts via gravity, is weaker than the weak nuclear force, and contains mass. It is estimated that the universe is composed of less than 5% atoms which are part of the Standard Model, 26.8% dark matter, and 68.3% dark energy.