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?

  • $\begingroup$ Do you mean "should" (which suggests some theoretical underpinning) as in the last line of your post or do you mean why do we know they have supermassive black holes as per your title? These are two different questions and I suggest the former does not have a definitive answer. $\endgroup$ – Rob Jeffries Oct 1 '17 at 17:59
  • $\begingroup$ Not all galaxies have supermassive black holes, but most of them, yes. $\endgroup$ – peterh Oct 1 '17 at 18:44
  • $\begingroup$ @RobJeffries, We have barely looked into our milkyway's center let alone rest of the 200 billion galaxies, But Scientist do believe that they all must have super massive black holes at their centers, My question was if the gravitational pull of these black holes is not enough to hold the galaxies together, what are they doing in the center?, hence challenging the popular assumption that all galaxies should have black holes at their center, I'm sorry if i confused you $\endgroup$ – Bad_Coder Oct 5 '17 at 15:07
  • $\begingroup$ I am still confused. It is not an "assumption" or a "belief" that most (large) galaxies have a supermassive black hole at their centre, it is an observation. $\endgroup$ – Rob Jeffries Oct 5 '17 at 15:29

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