1Today we confront what is arguably one of the most profound and persistent puzzles in modern astrophysics: the elusive nature of dark matter.
2The story begins with galaxy rotation curves, measurements of how quickly stars orbit at different distances from the centre.
3According to Newtonian mechanics, stars in the outer regions should orbit more slowly than those nearer the centre.
4Instead, observations consistently revealed flat rotation curves, implying that visible matter accounts for only a small fraction of the total mass.
5What I want to emphasize here is that dark matter neither emits nor absorbs light; we detect it purely through gravity.
6Contrary to a widespread misconception, dark matter is not simply ordinary matter hidden in dust clouds or dim stars.
7It was not the luminosity but the gravitational lensing of distant galaxies that provided the most compelling independent evidence.
8The leading candidates were originally expected to be detected within a decade, though decades of experiments have yielded no definitive signal.
9Some researchers proposed modifying the laws of gravity itself, yet such theories still struggle to account for observations of colliding galaxy clusters.
10Let me give you a concrete example: in the Bullet Cluster, the mass revealed by lensing is clearly separated from the visible gas.
11To sum up, dark matter remains undetected in the laboratory, yet its gravitational fingerprint is written across the entire cosmos.