However, when large amounts of dark matter aggregate into concentrated regions, its net gravitational influence is substantial, leading to measurable influences on stars and on nearby galaxies. Dark matter affects the expansion of the Universe, the path of light rays passing to us from distant objects, the orbits of stars around the centers of galaxies, and many other measurable phenomena in ways that convince us of its existence. We know about dark matter—and indeed it does exist—because of these measurable gravitational effects.
