Let's first have a look at the observed speed within galaxies (source): measure of speed within galaxy

On the left side we can see the speed of celestial bodies within galaxies derived by gravity only (gravity is usually created by a black hole at the center of a galaxy), on the right side we can see the observed speed of celestial bodies, both in respect of the distance from the galactic center.

As a result, the strength of dark matter (or "space force") could be calculated as:

$F$dark matter = $m$galaxy $\cdot$ $v$celestial body within galaxy $\frac{d}{dt}$ - $g$black hole, center of galaxy

Is this a viable calculation? And if so, are there any a graphs online which show this strength of dark matter (or "space force") in respect of the distance from the galactic center?

(Note: In this formula, a galaxy would be assigned a mass $m$galaxy much higher than the mass of its black hole. However, mathematically speaking one could also just decrease $dt$, so with a celestial body wandering from the center to the outskirts of the galaxy, $dt$ would decrease, hence the speed of time would increase. I doubt this is true because it would change the basic definition of speed $v$celestial body within galaxy as well and introduce all kinds of illogical contradictions, but in math everything is possible as we know from theory of relativity...)


  • $m$galaxy $\cdot$ $v$celestial body within galaxy $\frac{d}{dt}$ $[kg·\frac{m}{s^2}]$ = observable force (shown in picture on the right)
  • $g$black hole, center of galaxy $[kg·\frac{m}{s^2}]$, the force caused by gravity of a black hole (shown in picture on the left)
  • $F$dark matter $[kg·\frac{m}{s^2}]$, the force of dark matter up until a point in space
  • $\begingroup$ Please explain your notation. What do $F_{\text{dark_matter}}$, $dt$ and $G_{\text{black hole}}$ mean ? Without knowing what your terms mean, it is not even clear that your formula is dimensionally correct. $\endgroup$
    – gandalf61
    Oct 6, 2020 at 13:39
  • $\begingroup$ $mvd/dt$ is the force that is supposed to be caused by dark matter up until a point within the galaxy (shown in the right picture). $G_{black−hole}$ is the force that is caused by the black hole's gravity (shown in the left picture). $F_{dark-matter}$ is the difference... $\endgroup$
    – Marcus
    Oct 6, 2020 at 14:19
  • $\begingroup$ Hope that addendum will clarify things... $\endgroup$
    – Marcus
    Oct 6, 2020 at 14:29
  • 1
    $\begingroup$ "Note that the gravity usually comes from a black hole at the center of a galaxy)". No it doesn't. $\endgroup$
    – ProfRob
    Oct 6, 2020 at 19:27
  • $\begingroup$ @Rob: Changed some notations, I think it's pretty clear now. $\endgroup$
    – Marcus
    Oct 7, 2020 at 7:50

1 Answer 1


Your equation seems to be trying to express the gravitational contribution of dark matter as the difference between the observed gravitational force on a star at a given distance from the centre of a galaxy and the theoretical gravitational force based on the stars that can be seen in the galaxy.

This is pretty much what the velocity/distance curves show. The difference between observed and theoretical gravitational force can be worked out from the rotational velocity at a given distance because the centripetal force required to keep a star in a galactic orbit with speed $v$ and radius $r$ is proportional to $\frac {v^2}{r}$.

The super-massive black hole which lies, we believe, at the centre of most galaxies makes only a very small contribution to the gravitational attraction of the galaxy as whole. The typical mass of such a black hole is around $1 \%$ of mass of the stars in the galaxy, or around $0.1 \%$ of the total galactic mass including dark matter.


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