Close its mouth at the speed of light?
Close its mouth at the speed of sound?
Close its mouth faster than the speed of light?
Also… wouldn’t it be heavier than the galaxy itself? So it would create its own gravitational pull that’s like extremely strong?
No, impossible
No sound in space (and speed of sound depends on the material)
No, impossible
Very much heavier, a Galaxy is mostly empty space. If that thing is a single connected body it would probably just collapse into a black hole.