(Galaxy) Clusters


"Mountain peaks" peaks of the Large Scale Structure - Huge invisible dark matter potential well containing
Galaxies  - seen in visible light (left image)
Hot gas (shock heated by infall) - seen in X-rays (center image)
Hot gas up-scatters CMB photons to higher energy (order 1% for massive galaxy cluster)
Makes a hole in the CMB when viewed in radio
Number as a function of redshift very sensitive probe of Dark Energy