The gravitational pull of the combined masses of all the component stars and planets in a spiral galaxy must fall-off away from the centre, simply because there are less of them than in the tightly packed centre. So, before the 1970s, it had been assumed that stars which were further away from the centre of the galaxy would be rotating (around the central axis of the galaxy) at much slower speeds than those towards the centre.

But, in the 1970s, astronomers Vera Rubin and Albert Bosma published observational data which showed that the rotation speed of stars in (many) spiral galaxies appears to be roughly constant - from the centre to the outermost stars, and well beyond their optical discs. See :Galaxy Rotation Problemplugin-autotooltip__plain plugin-autotooltip_bigGalaxy Rotation Problem

"The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve.

Those new observations, that the rotation speed was constant, led to the idea of Dark Matterplugin-autotooltip__plain plugin-autotooltip_bigDark Matter

"The nature of the dominant component of galaxies and clusters remains unknown."

Source : Measuring the dark matter equation of state (Mon. Not. R. Astron. Soc. 415, L74–L77)"

In the 1930s, astronomical observations of galaxy rotations showed that the outer regions were rotating (about the galaxy's 'centre') at the same speed, or faster, than the central regions. Subsequent calculations referring to the galaxy's mass, and thus its internal gravitational attractions, showed that i… as a way of explaining the anomaly. By suggesting that 'missing mass' provided by the Dark Matter was apparently holding the galaxies together

With regard to our local galaxy - The Milky Way - new (2023) experimental data from the European Space Agency’s Gaia satellite appears to show that the outer-most stars are indeed revolving more slowly around the centre. Based on this, new calculations infer that the combined mass of the Milky Way is about four to five times less than had been suggested by several other earlier assessments. And so any 'Dark Matter' content would therefore also be considerably less than seems to be present in other galaxies.

There is currently no explanation for the observations.

Various possibilities have been suggested - including :

• The Milky Way may in some way differ structurally from other spiral galaxies
• For some reason there's only about half as much dark matter as in other similar galaxies
• There are problems with the latest measurements
• There are problems with previous measurements
• Over cosmological times and distances gravity might not be constant

See : Detection of the Keplerian decline in the Milky Way rotation curve Astronomy & Astrophysics manuscript no. AA:2023:47513 September 21, 2023