Andrew Robertson

The red dot in the James Webb Space Telescope image is the galaxy Bz5.3, whose light has traveled 12.5 billion years to reach us. It lies behind the Bullet Cluster, whose gravitational field acted as a lens and brightened it by a factor of 3.7.

The Bullet Cluster is a cosmic collision site where two vast clouds of galaxies, gas, and invisible matter slammed into each other. When light from distant stars passes near such a cluster, gravity bends its path like a lens. Astronomers study these distortions to work out where the mass is hiding.

Dark matter is invisible, but its gravity holds galaxies together. For a long time, it was thought that dark matter particles pass right through each other without noticing. However, in small galaxies, the density of dark matter in the center turns out to be lower than calculations predict.

A magnifying glass focuses sunlight into a bright point. If the glass has a tiny flaw, the spot of light becomes distorted. Gravitational lensing works in a similar way: a massive galaxy bends light rays from a more distant source, creating an enlarged and sometimes distorted image.