  We study the effect of the supersonic baryon--CDM flow, which has recently
been shown to have a large effect on structure formation during the dark ages
10 <~ z <~ 1000, on the abundance of luminous, low-mass satellite galaxies
around galaxies like the Milky Way. As the supersonic baryon--CDM flow
significantly suppresses both the number of halos formed and the amount of
baryons accreted onto such halos of masses 10^6 < M_{halo} / M_solar < 10^8 at
z >~ 10, a large effect results on the stellar luminosity function before
reionization. As halos of these masses are believed to have very little star
formation after reionization due to the effects of photo-heating by the
ultraviolet background, this effect persists to the present day. We calculate
that the number of low-mass 10^6 < M_{halo} / M_solar < 10^8 halos that host
luminous satellite galaxies today is typically suppressed by 50 percent, with
values ranging up to 90 percent in regions where the initial supersonic
velocity is high. We show that this previously-ignored cosmological effect
resolves most of the tension between the observed and predicted number of
low-mass satellites in the Milky Way, obviating the need for any other
mass-dependent star-formation suppression before reionization.
