Bahcall's work focuses on addressing questions such as: What is the large-scale structure of our Universe? How did structure form and evolve? How much dark matter exists in the Universe and where is it located? What is the nature of the mysterious dark energy? What is the fate of our Universe and its expansion? Bahcall uses different methods and a variety of tracers to answer these questions, including galaxies, clusters of galaxies, superclusters, and quasars. She combines observational data from large-scale surveys (such as the Sloan Digital Sky Survey and others) and other observations to determine the large-scale structure of the universe and its properties and compare it with those expected from cosmological simulations. Bahcall and collaborators’ determination of properties such as the cluster correlation function, the cluster mass function and its evolution with redshift, the mass-to-light function from galaxies to superclusters and from small to large cosmic scales, the geometrical shape of clusters and of large-scale structure have provided powerful constraints on cosmology including one of the first determinations of the mass-density of the Universe and the amplitude of mass fluctuations. Their results revealed a sub-critical Universe – containing only ~25% of the mass needed to halt the cosmic expansion; this result has now been nicely confirmed by recent observations and is part of the current standard LCDM cosmology.