  We suggest that the major fraction of binary mergers, which might provide
gravitational wave signal detectable by LIGO/Virgo, emerged from the hidden
mirror sector. Mirror particles do not interact with an ordinary observer
except gravitationally, which is the reason why no electromagnetic signals
accompanying gravitational waves from mergers with components composed of
mirror matter are expected. Therefore, if the dark matter budget of the
universe is mostly contributed by the mirror particles, we predict that only
about one binary neutron star (neutron star - black hole) merger out of ten,
observable by LIGO/Virgo, in particular at favorable conditions relative to the
line of sight, could be accompanied by prompt gamma ray burst and other
electromagnetic signals suitable for follow up observations. It seems the list
of candidate events recorded by LIGO/Virgo during third observational run
supports our predictions.
