Due to its proximity, M31 is the external giant-spiral galaxy that has been the most observed. Observations revealed the giant Stream dominating its halo, the past 2-3 Gyr star formation events, the stationary 10 kpc ring, and the unexpectedly large velocity dispersion of stars older than 2 Gyr. We have generated a dynamical model of M31 as a merger remnant that reproduces all the above properties, from the central bar to the halo. We find non-equilibrium and oscillatory motions in the gas of the M31 outskirts caused by instabilities due to the returning material from the merger that occurred 3 billion years ago. The model also reproduces the details of its HI extended rotation curve, including its 14 kpc bump and the observed increase of velocity beyond 25 kpc. This leads to a total dynamical M31 mass of 4.5e11 solar masses within 137 kpc after scaling it to the observed rotation curve.
This implies that one-third and two-third of the M31 mass are made of baryons and missing matter, respectively. This challenges cosmological expectations for which baryons account for less than one-sixth of the Universal matter mass. In other words, there is a deficit of missing matter in M31.
