The highest peaks in the cosmic density field host large clusters of galaxies that are
observable over vast distances. Their abundance is a sensitive probe of the physics of structure formation, and in my talk I will explain how clusters can be used to study effects of neutrinos or look for signatures of modified gravity.
These massive overdensities are only a small part of the complicated large-scale structure in the universe today. Gravitational collapse turns the smooth initial conditions into the cosmic web we observe today, consisting of filaments that surround large void regions. Galaxy clusters form in the knots of this cosmic web, and in the second part of the talk I will outline how to connect their number to the non-Gaussian shape of the underlying dark matter field which is not captured by the power spectrum or related two-point statistics that are commonly used in cosmology.
