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Motivated by results implying that the constituents of dark matter (DM) mightbe collisional, we consider a cosmological (toy-) model, in which the DM itselfpossesses some sort of thermodynamic properties. In this case, not only can thematter content of the Universe be treated as a classical gravitating fluid ofpositive pressure, but, together with all its other physical characteristics,the energy of this fluid's internal motions should be taken into account as asource of the universal gravitational field. This form of energy can compensatefor the extra (dark) energy, needed to compromise spatial flatness, while thepost-recombination Universe remains ever-decelerating. At the same time (i.e.,in the context of the collisional-DM approach), the theoretical curverepresenting the distance modulus as a function of the cosmological redshift,{\mu}(z), fits the Hubble diagram of a multi-used sample of supernova Ia eventsquite accurately. A cosmological model filled with collisional DM couldaccommodate the majority of the currently-available observational data(including, also, those from baryon acoustic oscillations), without the needfor either any dark energy (DE) or the cosmological constant. However, as wedemonstrate, this is not the case for someone who, although living in aUniverse filled with self-interacting DM, insists on adopting the traditional,collisionless-DM approach. From the point of view of this observer, thecosmologically-distant light-emitting sources seem to lie farther (i.e., theyappear to be dimmer) than expected, while the Universe appears to be eitheraccelerating or decelerating, depending on the value of the cosmologicalredshift. This picture, which, nowadays, represents the common perception inobservational cosmology, acquires a more conventional interpretation within thecontext of the collisional-DM approach.

A conventional approach to the dark-energy concept