Can one black hole suck in another black hole? In the recent news, scientists at NASA have found “unprecedented” black hole cluster near Andromeda’s central bulge. I wonder why doesn't all these black holes merge and such each other in until just 1 remains? Is that even possible? Can one black hole suck in another black hole?
 A: It's a very common misconception that black holes suck in matter. Outside the event horizon the gravitational field from a black hole is just like the gravitational field from normal matter with the same mass. The black hole can't suck in matter because matter will orbit it just like matter orbits a normal star.
It's likely there are a lot of black holes in Andromeda's central bulge simply because there are lots of old massive stars in Andromeda's central bulge and this type of star tends to form a black hole as it ages. The black holes orbiting the galaxy core won't fall into it unless there is some way they can lower their angular momentum. This can happen if they pass close to other stars. Interactions between two orbiting bodies can lower the angular momentum of one body and raise the angular momentum of the other. In a galaxy this is known as dynamical friction, and it tends to make heavy objects move inwards and light objects move outwards. Eventually the effect will cause many of the black holes to merge with the central black hole, but the timescale is much longer than the current age of the galaxy.
See Are galactic stars spiraling inwards? for some more info on this effect.
