  Masses of the ground and excited (1P, 2S, 1D, 2P, 3S) states of the
fully-heavy tetraquarks, composed of charm ($c$) and bottom ($b$) quarks and
antiquarks, are calculated in the diquark-antidiquark picture within the
relativistic quark model based on the quasipotential approach and quantum
chromodynamics. The quasipotentials of the quark-quark and diquark-antidiquark
interactions are constructed similarly to the previous consideration of mesons
and baryons. Relativistic effects are consistently taken into account. A
tetraquark is considered as a bound state of a diquark and an antidiquark. The
finite size of the diquark is taken into account, using the form factors of the
diquark-gluon interaction. It is shown that most of the investigated states of
tetraquarks lie above the decay thresholds into a meson pair, as a result they
can be observed only as broad resonances. The narrow state X(6900) recently
discovered in the di-$J/\psi$ production spectrum by the LHCb, CMS and ATLAS
Collaborations corresponds to an excited state of the fully-charmed tetraquark.
Other recently discovered exotic heavy resonances X(6200), X(6400), X(6600),
X(7200), X(7300) can also be interpreted as the different excitations of the
fully-charmed tetraquark.
