https://www.sciencedaily.com/releases/2021/10/211006160102.htm > For more than 2,000 years, scientists thought the atom was the smallest particle possible. Then, they discovered that it has a nucleus made up of protons and neutrons surrounded by electrons. After that, they found that the protons and neutrons themselves have a complex inner world full of quarks and antiquarks held together by a superglue-like force created by gluons. > "Protons along with neutrons constitute over 99 percent of the visible universe, meaning everything from galaxies and stars to us," said Yong Zhao -- a physicist at the U.S. Department of Energy's (DOE) Argonne National Laboratory. "Yet, there is still much we do not know about the rich inner life of protons or neutrons."