  [Abridged] 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 $\rho_{Macho} = (1-5) \times 10^9 h \msun
\Mpc^{-3}$, which in terms of the critical density corresponds to
$\Omega_{Macho}=(0.0036-0.017) h^{-1}$. Such a mass density is comparable to
the 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 $\Omega_{Macho}/\Omega_{Baryon} \sim 0.7 h$. However, the
cumulative uncertainties in the density determinations only require that
$\Omega_{Macho}/\Omega_{Baryon} \geq 1/6 h f_{gal}$, 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. We explore the addition constraints that arise if the Machos are white
dwarfs as suggested by the present microlensing data. We discuss the challenges
this scenario presents at both the local and cosmic scales, emphasizing in
particular the constraints on the required mass budget and nucleosynthesis
products (particularly carbon).
