Galaxy clusters are great labs for cosmology.
The high temperature means that galaxy clusters are very active. The bright X-ray radiation emitted by the galaxy clusters also makes them very easily observable. Galaxy clusters are best studied by satellites, which have views that are not obscured by Earth's atmosphere. Gravitational Lensing Gravitational lensing is the phenomena where mass warps space-time causing distortion of background galaxy images. When the CMB is scattered, it leaves behind a cold spot in the sky.