  Modern theories of physics tell that the vacuum is not an empty space. Hidden
in the vacuum is a structure of anti-quarks $\bar{q}$ and quarks $q$. The
$\bar{q}$ and $q$ pair has the same quantum number as the vacuum and
condensates in it since the strong interaction of the quantum chromodynamics
(QCD) is too strong to leave it empty. The $\bar{q}q$ condensation breaks the
chiral symmetry of the vacuum. The expectation value $<\bar{q}q>$ is an order
parameter. For higher temperature or higher matter-density, $|<\bar{q}q>|$
decreases reflecting the restoration of the symmetry. In contrast to these
clear-cut arguments, experimental evidence is so far limited. First of all, the
$\bar{q}q$ is nothing but the vacuum itself. It is neither visible nor
perceptible. In this article, we unravel this invisible existence by high
precision measurement of pionic atoms, $\pi^-$-meson-nucleus bound systems.
Using the $\pi^-$ as a probe, we demonstrate that $|<\bar{q}q>|$ is reduced in
the nucleus at 58% of the normal nuclear density by a factor of 77 $\pm$ 2%
compared with that in the vacuum. This reduction indicates that the chiral
symmetry is partially restored due to the extremely high density of the
nucleus. The present experimental result clearly exhibits the existence of the
hidden structure, the chiral condensate, in the vacuum.
