What if everything we can see in the universe—stars, planets, galaxies, and even ourselves—makes up less than 5% of reality?

In this episode of Astrophysics for All, I sit down with Professor Geraint Lewis, a cosmologist at the Sydney Institute for Astronomy, to explore the invisible universe. Originally from Wales, Professor Lewis completed his undergraduate degree at the University of London and went on to earn his PhD in astrophysics from the University of Cambridge’s Institute of Astronomy. He has worked at research institutions in the United States and Canada before joining the University of Sydney, where he now leads the Gravitational Astrophysics Group.

Although everything we can see—from stars to galaxies to ourselves—makes up less than 5% of the cosmos, the rest is dominated by mysterious dark matter and dark energy that scientists are still struggling to understand.

Professor Lewis explains how astronomers use gravitational lensing to map invisible matter, how galaxies grow by tearing apart smaller ones in a process known as galactic cannibalism, and why studying these phenomena can reveal the hidden structure of the universe. Along the way, Professor Lewis shares what life as a cosmologist looks like and offers practical advice for students curious about pursuing physics and astrophysics.