  The frequent detection of binary mergers of $\sim 30 M_{\odot}$ black holes
(BHs) by the Laser Interferometer Gravitational-Wave Observatory (LIGO)
rekindled researchers' interest in primordial BHs (PBHs) being dark matter
(DM). In this work, we looked at PBHs distributed as DM with a monochromatic
mass of $30 M_{\odot}$ and examined the encounter-capture scenario of binary
formation, where the densest central region of DM halo dominates. Thus, we paid
special attention to the tidal effect by the supermassive black hole (SMBH)
present. In doing so, we discovered a necessary tool called loss zone that
complements the usage of loss cone. We found that the tidal effect is not
prominent in affecting binary formation, which also turned out insufficient in
explaining the totality of LIGO's event rate estimation, especially due to a
microlensing event constraining the DM fraction in PBH at the mass of interest
from near unity to an order smaller. Meanwhile, early-universe binary formation
scenario proves so prevailing that the LIGO signal in turn constrains the PBH
fraction below one percent. Thus, people should put more faith in alternative
PBH windows and other DM candidates.
