Like light matter, dark matter attracts all matter gravitationally, and so it sustains a gravitational field. As astronomers study a galaxy, they can infer the presence of dark matter if the galaxy’s motion requires gravitational interactions greater than those that the galaxy’s light matter should allow.
On what do scientists base their understanding of dark matter?
Scientists infer the existence of dark matter from how gravity operates on light matter. Thus their understanding of dark matter is theoretical.