If so, the effective mass of the universe would reduce over time, presumably giving a similar effect. (newscientist.com)
Dark matter makes up most of the total mass of the universe. (webbtelescope.org)
This occurs at the point where the traveling twin reverses the direction of his motion and where his acceleration — not only with respect to the twin «at rest,» but also with respect to the large masses of the universe — takes place. (religion-online.org)