Credit: NASASpace messes with your stem cells and accelerates agingThe cells at the center of the problem are known as hematopoietic stem cells and progenitor cells. According to the study, HSCs decline in fitness post-spaceflight and show multiple signs of accelerated aging. Stem cells are found throughout the body, and they can do all sorts of things thanks to differentiation. You can think of a stem cell as a blank canvas waiting to be a painting. “The dark genome is composed of repetitive elements that represent approximately 50% of the human genome.