Dark matter, which makes up about 85% of the mass in the Universe, is more than simply dark. True, it can't shine by its own light, like hot coals, or even reflect light, like clouds or water. Neither electricity nor magnetism affect it. Currently, the only clues it gives us are through the gravitational pull it exerts on the normal mass that makes up the objects we can see, like stars and planets—and us.
Scientists first began to suspect the presence of some unknown type of matter in the 1930s when observations suggested that galaxies in clusters behaved as though the gravity pulling them was stronger than could be accounted for by the amount of matter the scientists could see. Strong evidence for the existence of dark matter finally arrived in the mid-2000s in the form of the Bullet Cluster, shown here, which is actually two clusters of galaxies colliding with each other. Observations using a technique called gravitational lensing showed that most of the mass in the cluster was well-separated from the hotly glowing—and thus visible—clouds of gas (pink), demonstrating that the dark matter (blue) did not feel pressure from the normal matter.
Gravitational lensing is still our best tool for finding and studying dark matter. It uses Albert Einstein's prediction, now demonstrated thousands of times, that concentrations of matter can bend light. During gravitational lensing, rays from a far-distant source, such as a young galaxy, passes by a concentration of matter, such as another galaxy, that lies between the source and the Earth. This concentration of matter serves as a lens, bending the light toward us and magnifying the source.
Strong lensing, the best-known type of gravitational lensing, can actually create several recognizable images of the source, while weak lensing, a more subtle phenomenon, causes distortions in the appearance of the more distant objects. The two techniques in tandem can not only help us understand dark matter, but enable us to use the mysterious substance as a tool to track the growth and evolution of the Universe.
But strong lenses are rare; weak lenses are more common but their effects are difficult to see without huge amounts of data about galaxy sizes and shapes to enable us to tell the true gravitational distortions from natural anomalies in shape. Finding enough gravitational lenses to constrain the properties of dark matter structures requires a powerful telescope with a huge field of view—like Rubin Observatory.
Rubin Observatory will find thousands more gravitational lenses of all sizes and configurations, and what these lenses show us about themselves as well as the objects they magnify will expand our understanding of the Universe in both time and space.