  We consider the operation of contraction of unitary irreducible
representations of the de Sitter group $ SO(4,1) $. It is shown that a direct
sum of unitary irreducible representations of the Poincar\'{e} group with
different signs of the rest mass is obtained as a result of contraction. The
results obtained are used to interpret the phenomena of "dark matter" and "dark
energy" in terms of the elementary quantum systems of de Sitter's world.
