In this chapter, we argue that if we are blinded by the constant stream of astrophysical and cosmological observations, we may forget that cosmology is the youngest of all the physical sciences. The 1965 discovery of the CMB radiation by Penzias and Wilson moved cosmology to the territory of firmly observational science from the domain of exclusively mathematical modeling, and the 1977 measurements of CMB’s anisotropies with detectors mounted on US spy aircraft opened its Big Science phase. A number of measurements of the CMB spectral shape by detectors mounted on rockets and balloons following the 1965 discovery led to fluctuating agreement with the values of the black body radiation spectrum. In particular, 1978–1979 measurements exhibited discrepancies that gave new impetus to the alternative explanations of the radiation. A series of satellite measurements since the early 1990s, with equipment similar to previous experiments but without atmospheric disturbances, led to the final phase of the convergence to the Hot Big Bang model.

Rudimentary catalogues of cosmic voids were first complied in the mid-1980s, but they were limited in scope by the lack of adequate deep galaxy survey data. Over several decades, catalogues have improved as have cosmic void identification methods. Voids in the galaxy distribution have become important objects and modern tools that are now used to investigate properties of the Universe. They have been and continue to be applied to problems in precision cosmology. The first step in utilizing this new tool is to compile massive surveys of the distant Universe that yield sufficiently large samples of cosmic voids. Then reliable void identification techniques were developed. These include sophisticated methods of 3D analysis. For some tests, “stacked voids” are created to enhance the measurement precision. Specific research results are summarized showing concrete results. Three other topics are discussed: topology of the void and supercluster structure, the LTB Universe models, and finally void galaxies.