Close its mouth at the speed of light?

No, impossible

Close its mouth at the speed of sound?

No sound in space (and speed of sound depends on the material)

Close its mouth faster than the speed of light?

No, impossible

Also… wouldn’t it be heavier than the galaxy itself? So it would create its own gravitational pull that’s like extremely strong?

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.

to reply to the question posed within the meme itself:

using the same physics which allow the creature to exist, i would create a device that consists of reflecting and refracting matter, like prisms, echo chambers, mirrors and lenses, with the overall capability to reflect and project a distorted ‘circus mirror’ holograph version of the creature at a subatomic level, which appears to be an order of magnitude larger, directly behind it.

then, when it turns around to face its new illusory foe, i would tie its shoelaces together.

• The speed of light is pretty slow at that scale, it would take thousands of years until the mouth is closed.

• The speed of sound (in air?) is much much slower than the speed of light. See above.

• How fast? I guess die, because that would cause a whole lot of other problems.

• Yeah, the gravity would pull the galaxy to it, but it would also take quite some time (one normal galactic year is about 225 million earth years, we wouldn’t instantly be pulled with an immense acceleration, we would probably just continue our orbit in a slightly different path or reverse, which would have it’s effects but not immediate in our scale, right?)