Editing Dark matter (section)

=== Galaxy rotation curves ===
{{Main|Galaxy rotation curve}}
[[File:Comparison of rotating disc galaxies in the distant Universe and the present day.webm|thumb|Animation of rotating disc galaxies. Dark matter{{snd}}shown in red{{snd}}is more concentrated near the center and it rotates more rapidly.]]
The arms of [[spiral galaxies]] rotate around their galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then the galaxy can be modelled as a point mass in the centre and test masses orbiting around it, similar to the [[Solar System]].<ref group=lower-alpha>This is a consequence of the [[shell theorem]] and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).</ref> From [[Kepler's Third Law]], it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.<ref>{{cite journal |last=Salucci |first=P. |title=The distribution of dark matter in galaxies |journal=[[The Astronomy and Astrophysics Review]] |date=2019 |volume= 27 |issue= 1 |pages= 2 |doi=10.1007/s00159-018-0113-1 |arxiv= 1811.08843 |bibcode= 2019A&ARv..27....2S}}</ref> Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases.

If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there may be a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.