  Conventional hadronic matter consists of baryons and mesons made of three
quarks and quark-antiquark pairs, respectively. The observation of a new type
of hadronic state, a doubly charmed tetraquark containing two charm quarks, an
anti-$u$ and an anti-$d$ quark, is reported using data collected by the LHCb
experiment at the Large Hadron Collider. This exotic state with a mass of about
3875 MeV$/c^2$ manifests itself as a narrow peak in the mass spectrum of
$D^0D^0\pi^+$ mesons just below the $D^{*+}D^0$ mass threshold. The near
threshold mass together with a strikingly narrow width reveals the resonance
nature of the state.
