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. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy.

Source : Wikipedia

It was first noted in the 1930s that the difference in the predicted rotation curve (using extrapolated star count estimates and the strength of thegravitational forceplugin-autotooltip__plain plugin-autotooltip_bigThe Gravitational constant

The gravitational attraction between two objects is dependent on the mass of the objects, the distance between them, and the gravitational constant (G).

The masses and distance can vary, but the constant, as the name implies, is always a fixed number.) correlated with data from observations, do not match. With errors typically in the range of 20 - 50%

Subsequent measurements have reliably found the same discrepancies. It was these observations that led to the hypothesis that there must be some kind of (invisible) gravitationally active material within galaxies that influences their rotation speed - so called 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…

Other theories are that Newtonian gravitational calculations do not (for unknown reasons) hold on a galactic scale, or, that gravity itself may not be constant.

Also see : Galaxy formationplugin-autotooltip__plain plugin-autotooltip_bigGalaxy formation

There are currently two (mutually exclusive) theories regarding the formation of galaxies, which according to current models, all have a supermassive Black Hole at their centre.

In brief, the theories are that :

1) The galaxy formed around a pre-existing Black Hole which was sufficiently massive as to attract all the nearby stars. and Galaxies (definition of)plugin-autotooltip__plain plugin-autotooltip_bigGalaxies (definition of)

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Although current astronomical techniques have revealed the presence of billions of entire galaxies, stretching to the 'edge' of the known universe, a strict definition of what a galaxy actually is has not been generally agreed.

" and Galaxy mass anomaliesplugin-autotooltip__plain plugin-autotooltip_bigGalaxy mass anomalies

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.

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