Final answer:
The galaxy is primarily dominated by dark matter because the gravity of visible matter alone cannot explain the observed gravitational effects. Dark matter is an invisible form of matter that can be inferred through its gravitational effects. Current models of galaxy formation and structure use cold dark matter as the main ingredient.
Step-by-step explanation:
The reason the galaxy is primarily dominated by dark matter according to the dynamical argument is because the gravitational effects observed in the galaxy, such as the high speeds of stars in their orbits, cannot be explained by the gravity of the visible matter alone.
Dark matter is an invisible form of matter that does not emit or interact with electromagnetic radiation, so it cannot be directly observed. However, its presence can be inferred through its gravitational effects on visible matter.
By measuring the rotation speeds and distributions of mass in galaxies, astronomers have determined that there is much more dark matter than visible matter in galaxies. The evidence heavily favors the existence of cold dark matter as the main ingredient in current models of galaxy formation and structure.