  We discuss the chemical evolution of metal poor galaxies and conclude that
their oxygen deficiency is not due to: the production of black holes by massive
stars or a varying slope of the Initial Mass Function, IMF, at the high-mass
end. A varying IMF at the low-mass end alone or in combination with: (a) an
outflow of oxygen-rich material, (b) an outflow of well-mixed material, and (c)
the presence of dark matter that does not participate in the chemical evolution
process, is needed to explain their oxygen deficiency. Outflow of material rich
in oxygen helps to account for the large helium to oxygen mass ratio values
derived from these objects, but it works against explaining the carbon to
oxygen mass ratio and the heavy elements minus carbon and oxygen to oxygen mass
ratio values.
