The most underdense regions of the Universe are the home of a population of gas-rich and low-metallicity dwarf galaxies. These galaxies could be the key to understand the earlier stages of hierarchical assembly within the ΛCDM. These galaxies present the exciting puzzle of how their stellar masses assembled — through gas accretion from the IGM, mergers, or a combination of both? The few observational and theoretical studies dedicated to this topic show conflicting results; during this discussion, I will outline an ongoing project that aims to address these questions by analyzing their neutral gas content and metallicity gradients, providing new insights into the assembly history of these galaxies.