The image shows such a massive accumulation that it manages to deform the space around it! The core is located in the center of the frame and is referred to as RCS2 J2327.
Such massive objects significantly affect the environment, so they actually change the nearest space. This effect is called gravitational lensing, which can cause bending, distortion and amplification of light from more distant objects. With it, you can view distant galaxies.
Gravitational lensing was predicted by Albert Einstein and is observed in three modes: strong, weak, and microlensing. Strong forms amazing images of distorted galaxies, wide arcs and cosmic phenomena, like Einstein's rings. But the weak appears more static, but allows us to calculate the masses of objects. Here is a general observation from the Hubble telescope and the HAWK-I instrument on the Very Large Telescope. The analysis showed that the massiveness of RCS2 J2327 reaches two quadrillion solar. Use the mouse to view each part of the cluster in two observations.