[erik_landerholm]

https://www.sciencedaily.com/releases/2018/10/181002102723.h...

“Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe. Primordial black holes could only have been created within the first milliseconds of the Big Bang as regions of the universe with a concentrated mass tens or hundreds of times that of the sun collapsed into objects a hundred kilometers across.”

“An as-yet unpublished reanalysis by the same team using an updated list of 1,048 supernovas cuts the limit in half, to a maximum of about 23 percent, further slamming the door on the dark matter-black hole proposal.”

[Entangled]

So, how dense you think the black hole at the center of the milky way is? Twice the whole galaxy at least? Or just a tiny fraction?

[GW150914]

If you really meant dense then you have to rememberer that the average density of a black hole enjoys an inverse proportionality to its volume. In other words a supermassive black hole is far less dense on average than a much smaller hole. However it is still far far denser than a whole galaxy, which is mostly empty space and diffuse dust.

I don’t think you meant dense though, I think you meant mass, because that’s sort of how your post seems to be phrased. The answer then is that it is a minute fraction of the mass of the galaxy. Sag A* is about 4 million solar masses, and the Milky Way has 250 billion stars, many of which are far more massive than Sol. Even ignoring other objects like stellar remnants and nebulae, the mass of the central black hole is negligable on the scale of the whole galaxy.