  Masses of the ground-state teraquarks composed from heavy $c$ and $b$ quarks
and antiquarks are calculated in the diquark-antidiquark picture in the
framework of the relativistic quark model based on the quasipotential approach.
The quasipotentials of the quark-quark and diquark-antidiquark interactions are
constructed similarly to the previous consideration of mesons and baryons. It
is assumed that the diquark and antidiquark interact in the tetraquark as a
whole and the internal structure of the diquarks is taken into account. All
such tetraquarks are found above the thresholds of decays to two heavy
quarkonia. This is a result of the consideration of the diquark not to be a
point-like object. Therefore such tetraquarks can be observed as broad
structures decaying dominantly to quarkonia. The broad structure next to the
di-$J/\psi$ mass threshold, recently observed by the LHCb Collaboration, can
correspond to the ground $2^{++}$-state tetraquark consisting of four charm
quarks.
