The supermassive black hole at the center of our galaxy, Sagittarius A*, is incredibly massive, but it does not contain most of the mass of the Milky Way galaxy.

Here's why:

* Sagittarius A* is estimated to be about 4 million times the mass of our sun. While that's a huge amount of mass, it's dwarfed by the total mass of the Milky Way.

* The Milky Way galaxy is estimated to have a total mass of around 1.5 trillion times the mass of our sun. This includes stars, gas, dust, dark matter, and everything else within the galaxy.

* Dark matter makes up the majority of the Milky Way's mass. While we can't directly observe it, its gravitational influence on visible matter tells us it accounts for a significant portion of the galaxy's mass.

So, while the supermassive black hole at the center is a massive object, it's just a tiny fraction of the total mass of our galaxy.