  In the energy domain of 1-2 GeV per nucleon, HADES has measured rare
penetrating probes (e+e-) in C+C, Ar+KCl, d+p, p+p and p+Nb collisions. For the
first time the electron pairs were reconstructed from quasi-free n+p
sub-reactions by detecting the proton spectator from the deuteron breakup. An
experimentally constrained NN reference spectrum was established. Our results
demonstrate that the gross features of di-electron spectra in C+C collisions
can be explained as a superposition of independent NN collisions. On the other
hand, a direct comparison of the NN reference spectrum with the e+e- invariant
mass distribution measured in the heavier system Ar+KCl at 1.76 GeV/u shows an
excess yield above the reference, which we attribute to radiation from
resonance matter. Moreover, the combined measurement of di-electrons and
strangeness in Ar+KCl collisions has provided further intriguing results which
are also discussed.
