Dekel is known for his contributions to research in cosmology, especially the study of the formation of galaxies and large-scale structure in the Universe, which is dominated by dark energy and dark matter. His expertise is dwarf galaxies and supernova feedback (1986, 2003), large-scale cosmic flows and early estimates of fundamental cosmological parameters (1989-2001), the structure of dark-matter galactic halos (2000–2003), and the theory of galaxy formation (2003–2012).