Scientists are now largely convinced that there is indeed one at the centre of the galaxy... the evidence indicates a compact high-mass object there; if it's not a black hole, we don't know what else it could be.
No - The volume of the Milky Way galaxy is larger than the volume of its host black hole. The accumulated mass of the Milky Way galaxy is greater than the mass of its host black hole. The density of the Milky Way galaxy is much smaller than the density of its host black hole.
The middle of the milky way is a black hole and a black hole cannot be made up as matter. The middle of the milky way has no size, but the black hole sucks the light making it look big in pictures. The actual size is nothing.
It is suggested that there is a black hole in the center of not just The Milky Way, but most other galaxies.
Fortunately, no.
Black holes are common in most galaxies. It is not odd that there would be a black hole in the middle of the Milky Way.
At the centre of our Galaxy, the Milky Way
A supermassive black hole.
They don't specifically orbit the black hole. Stars orbit the Milky Way because in general, they are attracted to the other masses in the Milky Way. The supermassive black hole is only a tiny fraction of the total mass of the Milky Way. Well, you might say that they move around the black hole, but that's only because it happens to be there. Without they black hole, they would move around the center of the Milky Way anyway.
All galaxies have black holes, even the Milky Way.
A supermassive black hole.
The galactic center of the Milky Way is a compact object of very large mass (named Sagittarius A), strongly suspected to be a supermassive black hole.
I believe the closest black hole is in the center of the Milky Way galaxy... But is a black hole a dark hole? Hmm.