We estimate the contribution of Massive Compact Halo Objects ( Machos ) and their stellar progenitors to the mass density of the Universe .
If the Machos that have been detected reside in the Halo of our Galaxy , then a simple extrapolation of the Galactic population ( out to 50 kpc ) of Machos to cosmic scales gives a cosmic density \rho _ { Macho } = ( 1 - 5 ) \times 10 ^ { 9 } h M _ { \odot } { Mpc } ^ { -3 } , which in terms of the critical density corresponds to \Omega _ { Macho } = ( 0.0036 - 0.017 ) h ^ { -1 } .
Should the Macho Halo extend out to much further than 50 kpc , then \Omega _ { Macho } would only be larger .
Such a mass density is comparable to the cosmic baryon density implied by Big Bang Nucleosynthesis .
If we take the central values of the estimates , then Machos dominate the baryonic content of the Universe today , with \Omega _ { Macho } / \Omega _ { B } \sim 0.7 h .
However , the cumulative uncertainties in the density determinations only require that \Omega _ { Macho } / \Omega _ { B } \geq { 1 \over 6 } hf _ { gal } , where the fraction of galaxies that contain Machos f _ { gal } > 0.17 and h is the Hubble constant in units of 100 km s ^ { -1 } Mpc ^ { -1 } .
Our best estimate for \Omega _ { Macho } is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman- \alpha forest ; however , measurements of \Omega _ { { Ly } \alpha } are at present uncertain , so that such a comparison may be premature .

If the Machos are white dwarfs resulting from a single burst of star formation ( without recycling ) , then their main sequence progenitors would have been at least twice more massive : \Omega _ { \star } = ( 0.007 - 0.034 ) h ^ { -1 } .
Thus , far too much gaseous baryonic material would remain in the Galaxy unless there is a Galactic wind to eject it .
Indeed a Macho population of white dwarfs and the gas ejected from their main sequence progenitors accounts for a significant fraction of all baryons .
This fact must be taken into account when attempting to dilute the chemical by-products of such a large population of intermediate mass stars .
We stress the difficulty of reconciling the Macho mass budget with the accompanying carbon production in the case of white dwarfs .
In the simplest picture , even if the excess carbon is ejected from the Galaxy by a Galactic wind , measurements of carbon abundances in Lyman \alpha forest lines with values 10 ^ { -2 } solar require that only about 10 ^ { -2 } of all baryons can have passed through the white dwarf progenitors .
Such a fraction can barely be accommodated by our estimates of \Omega _ { Macho } and would be in conflict with \Omega _ { \star } .