  Sound waves from the primordial fluctuations of the Universe imprinted in the
large-scale structure, called baryon acoustic oscillations (BAOs), can be used
as standard rulers to measure the scale of the Universe. These oscillations
have already been detected in the distribution of galaxies. Here we propose to
measure BAOs from the troughs (minima) of the density field. Based on two sets
of accurate mock halo catalogues with and without BAOs in the seed initial
conditions, we demonstrate that the BAO signal cannot be obtained from the
clustering of classical disjoint voids, but is clearly detected from
overlapping voids. The latter represent an estimate of all troughs of the
density field. We compute them from the empty circumsphere centers constrained
by tetrahedra of galaxies using Delaunay triangulation. Our theoretical models
based on an unprecedented large set of detailed simulated void catalogues are
remarkably well confirmed by observational data. We use the largest recently
publicly available sample of luminous red galaxies from SDSS-III BOSS DR11 to
unveil for the first time a >3$\sigma$ BAO detection from voids in
observations. Since voids are nearly isotropically expanding regions, their
centers represent the most quiet places in the Universe, keeping in mind the
cosmos origin and providing a new promising window in the analysis of the
cosmological large-scale structure from galaxy surveys.
