Caption: Galaxy rotation curve for M33 (AKA the Triangulum Galaxy) superimposed on a to-scale image of M33 (one supposes).

Features:

1. M33 is in the constellation Triangulum---which is why it is also called the Triangulum Galaxy).

   It is 0.73--0.94 Mpc away and has radial velocity -179(3) km/s.

   It is type SA(s)cd.

2. M33, the Andromeda Galaxy (M31), and the Milky Way are the only large galaxies in the Local Group of galaxies.

3. The graph's horizontal axis is radius from the galaxy center in kilo-light-years. Divide by 3 to get the approximate distance in kiloparsecs.

4. The graph's vertical axis is velocity in km/s (which is the natural unit for most astronomical-object velocities).

5. The upper curve is the actual observed galaxy rotation curve measured using stars and the hydrogen 21-centimeter line (21.1061140542 cm, 1420.4057 5176 67(9) MHz ≅ 1420 MHz).

6. The lower curve is what is expected just from the luminous matter: stars and interstellar medium (ISM).

7. The discrepancy between the two curves is qualitatively the same for almost all large galaxies and these discrepancies prove that there must of ∼> 30 times more mass than observed in baryonic matter (i.e., ordinary matter made of protons, neutrons, and electrons) in almost all large galaxies (e.g., Dekel et al. 2019, Fig. 1).

8. The invisible matter is called the dark matter. For an explication of dark matter, see Galaxies file: galaxy_rotation.html.