<p>Supermassive black holes are really small, compared to the mass of the galaxy.</p>

<p>We know how big the black hole is, because we can see the stars in the centre of galaxies orbiting it. So in a typical spiral galaxy the black hole has a mass of a few million suns.  But the galaxy has a mass of a few <em>trillion</em> suns.</p>

<p>The black hole dominates the gravity in the cluster at the very centre of a galaxy, but out by us, it is entirely negligible</p>

<p>You don't need to use general relativity for this.  Relativity is significant when you are close to a black hole, but if you are far from it, then ordinary Newtonian gravity is an excellent approximation.  The force of gravity drops in inverse proportion to the square of the distance, which means that something as small as a supermassive black hole is much too tiny to have any effect on the the orbital speeds of most of the galaxy. </p>

<p>To model the orbital speeds there has to be something with a mass of trillions of suns, which entirely surrounds the disk of the galaxy, which the stars can pass through without friction and is completely invisible.  Finding that is the challenge of dark matter.</p>

CC BY-SA 4.0
2020-04-22T12:23:42.363Z
Thank you James K that is a very succinct and complete explanation. Mike
Mike
CC BY-SA 4.0
2020-04-22T12:49:14.093Z
Re "To model the orbital speeds there has to be something with a mass of trillions of suns, which entirely surrounds the disk of the galaxy," -> OR we have got gravity wrong and it does not behave with a one size fits all law. Occam says this is the preferred solution. (Occam makes no comment as to whether the preferred solution is correct).
Russell McMahon
CC BY-SA 4.0
2023-09-19T00:51:37.730Z
@RussellMcMahon How is making the equation for gravity more complicated preferred by Occam? It's simpler to postulate matter that doesn't interact electromagnetically.
PM 2Ring
CC BY-SA 4.0
2023-09-19T05:33:27.027Z
@PM2Ring Occam says, choose: - Our understanding of the "laws" are improving. We can better tell you where Mercury is now than we could, but it doesn't work quite as we so far expect on larger scales.  OR - There is (Carl Sagan model) Invisible floating magical Unicorn mass out there. We cant see it but we can tell that it has mass so it MUST be there. || It's a trolley problem :-)
Russell McMahon
CC BY-SA 4.0
2020-04-22T06:14:49.233Z
2020-04-22T06:14:49.233Z
James K
<p>Don't think of black holes as fantastic objects that exert some kind of super-strong gravity. They do indeed do that, but only in zones close to the event horizon (i.e. only in their very immediate neighborhoods). If the Sun were replaced by a black hole of the same mass, the Earth would still continue to orbit as normal.</p>
<p>In the same way, the supermassive black holes at the center of galaxies simply acts as a supermassive body of a few million solar masses. The gravitational effects of that supermassive black hole on the rest of the galaxy are well approximated by Newton's laws. For the purpose of the dark matter problem, it doesn't matter if the body at the center of galaxies is a black hole, a star, etc - they all have the same effect on the outskirts of the galaxy.</p>

CC BY-SA 4.0
2020-04-22T06:53:02.727Z
2023-09-17T23:50:19.533Z
2023-09-17T23:50:19.533Z
Allure
<p>Besides the fact that the central black holes really aren't all that massive compared to the galaxy, there is another more fundamental reason they can't explain the overly rapid revolution of the outer stars. The problem is that the outer stars are moving faster than they should be <em>compared to the inner stars</em>. If you make the central black hole a trillion times more massive, the outer stars would have to be moving faster, but the inner stars would have to be moving a lot faster.</p>
<p>But if you spread out a bunch of mass throughout the galaxy, then the inner stars don't have to be going so much faster to stay in orbit, because there's only so much extra mass inside their orbit. But the outer stars are influenced by more mass and so are moving faster than you expect.</p>
<p>Smaller, widely-scattered <a href="https://www.frontiersin.org/articles/10.3389/fspas.2021.681084/full" rel="nofollow noreferrer">primordial black holes</a> are a dark-horse candidate for being the dark matter, or maybe a part of it, but that's considered unlikely at this point.</p>

CC BY-SA 4.0
2023-09-18T23:27:50.297Z
2023-09-18T23:27:50.297Z
Mark Foskey
<p>Please could some explain this to me - I understand that some galaxies are rotating with a speed that the stars on the outer reaches should not be held within the spiral given the observed matter and consequent gravitational field it forms. As a consequence of this, we are looking for dark matter that would compensate for this imbalance. But my question is this, how do we know that a black hole found at the centre of many galaxies is not providing the ‘extra’ gravitional curved space time required to hold the spiral galaxy together ?</p>

CC BY-SA 4.0
2020-04-22T12:23:45.097Z
Somewhat related https://astronomy.stackexchange.com/a/10153/9527
usernumber
CC BY-SA 4.0
2021-12-06T23:10:06.590Z
Hi, just curious; do you feel either answer can be accepted or is further clarification needed?
uhoh
<gravity><dark-matter>
The anomaly of spiral galaxies and the need for dark matter
Mike
# The anomaly of spiral galaxies and the need for dark matter

(Asked by: username_0 on 2020-04-22)

Please could some explain this to me - I understand that some galaxies are rotating with a speed that the stars on the outer reaches should not be held within the spiral given the observed matter and consequent gravitational field it forms. As a consequence of this, we are looking for dark matter that would compensate for this imbalance. But my question is this, how do we know that a black hole found at the centre of many galaxies is not providing the ‘extra’ gravitional curved space time required to hold the spiral galaxy together ?

**Tags:** gravity, dark-matter

> **Comments:**
> **username_1**: Somewhat related https://astronomy.stackexchange.com/a/10153/9527  
> **username_2**: Hi, just curious; do you feel either answer can be accepted or is further clarification needed?  

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## 3 answers

### Answer by username_3 on 2020-04-22. Score: 8

Supermassive black holes are really small, compared to the mass of the galaxy.

We know how big the black hole is, because we can see the stars in the centre of galaxies orbiting it. So in a typical spiral galaxy the black hole has a mass of a few million suns. But the galaxy has a mass of a few *trillion* suns.

The black hole dominates the gravity in the cluster at the very centre of a galaxy, but out by us, it is entirely negligible

You don't need to use general relativity for this. Relativity is significant when you are close to a black hole, but if you are far from it, then ordinary Newtonian gravity is an excellent approximation. The force of gravity drops in inverse proportion to the square of the distance, which means that something as small as a supermassive black hole is much too tiny to have any effect on the the orbital speeds of most of the galaxy.

To model the orbital speeds there has to be something with a mass of trillions of suns, which entirely surrounds the disk of the galaxy, which the stars can pass through without friction and is completely invisible. Finding that is the challenge of dark matter.

> **Comments:**
> **username_0**: Thank you username_3 that is a very succinct and complete explanation. username_0  
> **username_4**: Re "To model the orbital speeds there has to be something with a mass of trillions of suns, which entirely surrounds the disk of the galaxy," -> OR we have got gravity wrong and it does not behave with a one size fits all law. Occam says this is the preferred solution. (Occam makes no comment as to whether the preferred solution is correct).  
> **username_5**: @RussellMcMahon How is making the equation for gravity more complicated preferred by Occam? It's simpler to postulate matter that doesn't interact electromagnetically.  
> **username_4**: @PM2Ring Occam says, choose: - Our understanding of the "laws" are improving. We can better tell you where Mercury is now than we could, but it doesn't work quite as we so far expect on larger scales.  OR - There is (Carl Sagan model) Invisible floating magical Unicorn mass out there. We cant see it but we can tell that it has mass so it MUST be there. || It's a trolley problem :-)  

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### Answer by username_6 on 2020-04-22. Score: 2

Don't think of black holes as fantastic objects that exert some kind of super-strong gravity. They do indeed do that, but only in zones close to the event horizon (i.e. only in their very immediate neighborhoods). If the Sun were replaced by a black hole of the same mass, the Earth would still continue to orbit as normal.

In the same way, the supermassive black holes at the center of galaxies simply acts as a supermassive body of a few million solar masses. The gravitational effects of that supermassive black hole on the rest of the galaxy are well approximated by Newton's laws. For the purpose of the dark matter problem, it doesn't matter if the body at the center of galaxies is a black hole, a star, etc - they all have the same effect on the outskirts of the galaxy.


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### Answer by username_7 on 2023-09-18. Score: 1

Besides the fact that the central black holes really aren't all that massive compared to the galaxy, there is another more fundamental reason they can't explain the overly rapid revolution of the outer stars. The problem is that the outer stars are moving faster than they should be *compared to the inner stars*. If you make the central black hole a trillion times more massive, the outer stars would have to be moving faster, but the inner stars would have to be moving a lot faster.

But if you spread out a bunch of mass throughout the galaxy, then the inner stars don't have to be going so much faster to stay in orbit, because there's only so much extra mass inside their orbit. But the outer stars are influenced by more mass and so are moving faster than you expect.

Smaller, widely-scattered [primordial black holes](https://www.frontiersin.org/articles/10.3389/fspas.2021.681084/full) are a dark-horse candidate for being the dark matter, or maybe a part of it, but that's considered unlikely at this point.


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