Volume changes when mixing DMSO and water
If we assume that the density of the mixture is just the average of the individual densities, would get 1 litre of a mixture with a density of 1.04635 kg/l.
However, measured density (1, table 1) is 1.0682.

We thus expect to see a volume that is 0.9795 times the added individual volumes.

In this case: 0.9795 litres.

LeBel, R. G.; I. Goring, D. A. Density, Viscosity, Refractive Index, and Hygroscopicity of Mixtures of Water and Dimethyl Sulfoxide. J. Chem. Eng. Data, 1962, 7 (1), 100–101