Peebles named it the void phenomenon in 2001: the great voids of the local universe are far emptier than ΛCDM allows them to be. The model's hierarchical clustering fills voids with low-mass dark matter halos, roughly a factor of ten more dwarf-capable halos than the dwarf galaxies actually observed there (Tinker and Conroy 2009 framed the standard defense; arXiv analyses of SDSS voids confirm the deficit), and the observed void walls are sharper than simulations produce. Surveys of the Local Void and the larger SDSS voids keep finding pristine emptiness where the model expects a population of faint dwarfs, gas clouds, and stripped-down halos: the predicted inhabitants simply fail to appear.
The standard escapes invoke galaxy-formation physics: reionization and supernova feedback can keep the smallest halos dark, so the void halos may exist while their galaxies do not. But this defense is double-booked, since the same suppression must simultaneously solve the missing-satellites accounting in groups, and tuned so that void dwarfs are suppressed while equally small field dwarfs at void edges shine. The sharpness problem evades the escape entirely: making galaxies invisible does not evacuate the dark halos themselves, whose gravity should still soften void boundaries and generate a measurable lensing and dynamical presence that deep surveys do not find. The void phenomenon is thus a structure problem masquerading as a galaxy-formation problem.
The standing is durable: two decades of deeper surveys have sharpened rather than dissolved the emptiness, and Rubin-LSST's dwarf census in and around nearby voids will test the suppressed-galaxy defense directly, by either finding the dark population's lensing signature and occasional luminous stragglers, or confirming that the voids are empty of structure itself.