  Recently found accelarated expansion of our Universe is due to the presence
of a new kind of matter called "$\Lambda$ - field" or quintessence. The
limitations on its equation of state are found from the fact of its
unclustering at all scales much smaller than the cosmological horizon. It is
discussed how these limitations affect on the possibility to approximate the
accelarated expansion by such cosmological models as the model of Chaplygin
gas.
