Have you ever gazed at a starry sky far away from city lights? In the relative darkness of a rural setting, the night sky appears as a vast canvas, illuminated only by a handful of distant stars. What we observe in these twinkling lights are what scientists refer to as 'visible matter.' But did you know that this visible matter makes up only about 5% of the universe? The remaining 95% is composed of mysterious dark matter and dark energy, which, unlike stars and galaxies, don't emit or reflect light yet dominate the universe's evolution. Understanding dark matter matters because it reshapes our conception of the cosmos and hints at its future destiny.

Dark matter is a hypothetical form of matter that, unlike regular matter, does not emit or interact with electromagnetic radiation (like light), making it invisible and detectable only through its gravitational effects. We understand it exists because of its clear influence on the motion of galaxies and galaxy clusters. For instance, astronomers first suspected dark matter in the 1930s when studying galaxy clusters. They noticed that the mass inferred from visible light (the number of stars) was much less than the mass needed to explain the clusters' gravitational binding. In other words, galaxies spun faster than expected based on the visible matter, indicating that some unseen mass was also exerting gravitational forces. A classic example of dark matter in action can be observed in the rotation curves of galaxies, such as the Andromeda galaxy. Traditionally, based on the visible stars, we would expect that stars further from the center would spin slower, resembling how planets move in our solar system. However, measurements show that stars far from the center are actually orbiting at similar speeds as those closer in. This suggests a vast halo of dark matter enveloping the galaxy, providing the gravitational glue needed to hold everything together. Moreover, the phenomenon known as gravitational lensing, predicted by Einstein’s general theory of relativity, allows astronomers to visualize dark matter’s distribution. When light from distant galaxies passes by massive objects like galaxy clusters, it bends, creating distortions. By measuring this effect, scientists can infer the presence and amount of dark matter in those clusters.