There is accumulating evidence that the faint end of the galaxy luminosity function might be very different in different locations .
The luminosity function might be rising in rich clusters and flat in regions of low density .
If galaxies form according to the model of hierarchical clustering then there should be many small halos compared to the number of big halos .
If this theory is valid then there must be a mechanism that eliminates at least the visible component of galaxies in low density regions .
A plausible mechanism is photoionization of the intergalactic medium at a time before the epoch that most dwarf galaxies form in low density regions but after the epoch of formation for similar systems that ultimately end up in rich clusters .
The dynamical timescales are found to accommodate this hypothesis in a flat universe with \Omega _ { m } \lesssim 0.4 .

If small halos exist but simply can not be located because they have never become the sites of significant star formation , they still might have dynamical manifestations .
These manifestations are hard to identify in normal groups of galaxies because small halos do not make a significant contribution to the global mass budget .
However , it could be entertained that there are clusters of halos where there are only small systems , clusters that are at the low mass end of the hierarchical tree .
There may be places where only a few small galaxies managed to form , enough for us to identify and use as test probes of the potential .
It turns out that such environments might be common .
Four probable groups of dwarfs are identified within 5 Mpc and the assumption they are gravitationally bound suggests M / L _ { B } \sim 300 - 1200 ~ { } M _ { \odot } / L _ { \odot } , 6 \pm factor 2 times higher than typical values for groups with luminous galaxies .