  The optical/far-IR extragalactic background light (EBL) from both resolved
and unresolved extragalactic sources is an indicator of the total luminosity of
cosmic structures, as the cumulative emission from young and evolved galactic
systems, as well as from active galactic nuclei (AGNs), is recorded in this
radiation. This is a brief review of some of the implications of the observed
brightness of the night sky for the stellar mass density and average
metallicity of the universe today, and of the possible contribution of MACHO
progenitors and QSOs to the EBL. Assuming a Salpeter initial mass function with
a cutoff below 0.6 M_sun, a lower limit of Omega_{g+s}h^2>0.0015 I_{60} can be
derived to the visible (recycled gas + stars) mass density required to generate
an EBL at a level of I_{EBL}=60 I_{60} nW/m^2/sr: our latest, best-guess
estimate is Omega_{g+s}h^2 ~ 0.0023 I_{60}, which implies a mean metallicity at
the present-epoch of y_Z Omega_{g+s}/Omega_b ~ 0.2 Z_sun. If massive dark halos
around spiral galaxies are partially composed of faint, old white dwarfs, i.e.
if a non-negligible fraction (a few percent) of the nucleosynthetic baryons is
locked in the remnants (MACHOs) of intermediate-mass stars forming at very high
redshifts, then the bright early phases of such halos should contribute
significantly to the observed EBL. Assuming a standard black hole accretion
model for quasar activity and using recent observations of the quasar
population and new synthesis models for the cosmic X-ray background, we
estimate a present mass density of QSO remnants of rho_{BH} ~ 3e5 M_sun/Mpc^3
for a 10% efficiency of accreted mass-to-radiation conversion. The quasar
contribution to the brightness of the night sky is I_QSO ~ 2 nW/m^2/sr.
