I will discuss a new way to characterize voids and some practical applications of voids as probes of theories of modified gravity. During the first part, I will present the void boundary profile in which voids are described with respect to their boundary and not their ill defined centres. In contrast to haloes, voids are distinctly non-spherical and have most of their mass at their edge – such features are easily captured by the boundary profile but not by the customary spherical profile. When stacked with respect to the boundary, the void density profile is self-similar and depends on only one parameter, the width of the density ridge.
I will also explain why voids are, at least in theory, ideal probes for testing modified gravity models. Many such theories give rise to fifth forces, which, while screened in high density regions, are allowed to attain maximum values in under dense void-like regions. This leads to emptier voids and to matter and galaxies flowing out faster from within voids. These effects can be measured using void weak lensing and void redshift space distortions. Observationally, the former approach is challenging since voids produce only a small lensing signal. More promising is the use of weak lensing by line-of-sight tunnels, which are elongated regions along the line of sight that intersect several voids without passing trough over dense regions. The lensing by tunnels is more than an order of magnitude larger than that by voids and preserves the imprint of modified gravity models.