  The dominance of dark energy in the universe has necessitated the
introduction of a repulsive gravity source to make q0 negative. The models for
dark energy range from a simple lambda-term to quintessence, Chaplygin gas,
etc. We look at the possibility of how change of behaviour of missing energy
density, from DM to DE, may be determined by the change in the equation of
state of a background fluid instead of a form of potential. The question of
cosmic acceleration can be discussed within the framework of theories which do
not necessarily include scalar fields.
