Final answer:
Supermassive black holes, present at the centers of most galaxies, exert the gravitational force responsible for holding these galaxies together. These black holes' masses are much greater than stars and even neutron stars, and they play a pivotal role in the structure and evolution of galaxies, including the production of quasars.
Step-by-step explanation:
The gravitational force that holds most galaxies together is primarily due to the presence of supermassive black holes at their centers. Supermassive black holes can have masses millions to billions of times that of our Sun and are thought to have formed when matter first coalesced into galaxies billions of years ago. As galaxies evolved, these black holes became the anchors, exerting a strong gravitational pull that maintains the structure of the galaxy.
Observational evidence, including data from the Hubble Space Telescope, suggests that supermassive black holes exist at the cores of many galaxies, such as the Milky Way and the Andromeda galaxy. The energy associated with these black holes, particularly observed in quasars, verifies their immense influence on galaxy structure through energetic emissions and the dynamics of surrounding matter, such as accretion disks and ejecting jets of material.