Our own Milky Way appears as a great white band of light crossing the sky. What are seeing is not a giant white hiking trail in outer space but actually myriads of stars like our own, with most of
these stars too numerous and far away for our eyes to distinguish one from the other.
Our Milky Way is home to more than 100 billion stars which are all bound together by gravity. We think we understand how gravity works at the level of everyday life. For example, we know very well where and when a ball will fall when we throw it into the air, and interestingly most of us can do this computation in our heads from a very young age. Even better, if we apply these intuitive rules for gravity to our own Solar system, they also work. It was Galileo who first made the discovery of the ‘universality’ of gravity, and Kepler who quantified our understanding of how planetary orbits work close to 400 years ago. This is good news as it means we are able to predict with great accuracy where the planets will go from year to year. We also know something about the motion of planets around the Sun. For instance, we know that the planets farthest from the Sun will move more slowly than those closer to the Sun. Let us expand our thinking now not the motions of planets but to those of stars. Analogous to the planets, the stars in the Milky Way farthest from the center should move more slowly than those closest to the center. Rather surprisingly, instead we see that the bulk of the stars move at the same speed wherever they are situated with respect to the center. No one knows really why this should be the case. The best idea is that there is some massive component of the Milky Way which we cannot see when we look up at the night sky. We now call this component dark matter, and the search for dark matter is one of the great scientific questions of the 21st century.