The standard model of cosmology considers the existence of two components of
unknown nature, ``dark matter'' and ``dark energy'', which determine the
cosmological evolution. Their nature remains unknown, and other models can also
be considered. In particular, it may be possible to reinterpret the recent
cosmological observations so that the Universe does not contain two fluids of
unknown natures, but only one fluid with particular properties. After a brief
review of constraints on this unifying ``dark fluid'', we will discuss a
specific model of dark fluid based on a complex scalar fluid.Comment: 4 pages, 4 figures, to appear in the XXIst IAP Colloquium "Mass
Profiles and Shapes of Cosmological Structures", Paris 4-9 July 2005, France,
(Eds.) G. Mamon, F. Combes, C. Deffayet, B. Fort, EAS Publications Serie
