Le 10/11/2017 =C3=A0 08:10, Jos Bergervoet a =C3=A9crit=C2=A0:

We know what the rotation curves are, so the acceleration of
stars on average is known, and it is known that this does not
fit with the gravity of known matter in the galaxies. So you
do not need the observations as you describe here to get this
information. What we do *not* know is:
1) Is there more matter than the known matter, so stronger
gravity and therefore restoring agreement with the movement?

That would be a nice solution. And if space between the stars wasn't
empty but filled with some kind of very thin gas?

2) Is there another force that adds to the effect of gravity
so together they give agreement with the motion?
The first possibility leads to the hunt for dark matter, the
second to the search for a "fifth force".


Dark matter was supposed to be in some kind of "halo" outside the
galaxy. The stars then, are pulled by the outside. A fifth force would
have a vector centered in the center of the milky way.

Is it possible then, to figure out this from the orbit of a known star,
say, the sun?

The observations with telescopes as discussed above will not
help with these questions at all, they will just reproduce the
already observed disagreement between motion and the gravity
of known matter. Which then leaves us again with the same two
questions.


A fifth force would perturb the orbit of the sun in a different way than
matter in a halo. Besides, even if it is weak it has been there since
eons. After all this time (age of the sun around 5GY) some perturbation
should be observable.

Shouldn't an analysis of the orbit of the sun give an answer to that?

[[Mod. note --
Interstellar space is indeed filled with a "very thin gas":
https://en.wikipedia.org/wiki/Interstellar_medium
https://en.wikipedia.org/wiki/Intracluster_medium
This is already included in counts of "known matter".
-- jt]]