I know this has now been a common understanding that every galaxy has a supermassive black hole at their centers, But how does this understanding came into existence? Even though these massive black holes have million or even billion times mass of our sun, their mass is still nothing in compared to the rest of the stars in the galaxy. And it has been proven that the gravitational pull of these supermassive black holes isn't enough to form a galaxy (hold these millions of star together in a galaxy). Galaxies can form because of dark matter (still a speculation though).
So how does one come into a conclusion that every galaxy should have super massive black hole at their center?