  Cybele asteroids constitute an appealing reservoir of primitive material
genetically linked to the outer Solar System, and the physical properties of
the largest members can be readily accessed by large telescopes. We took
advantage of the bright apparition of (65) Cybele in July and August 2021 to
acquire high-angular-resolution images and optical light curves of the asteroid
with which we aim to analyse its shape and bulk properties. 7 series of images
acquired with VLT/SPHERE were combined with optical light curves to reconstruct
the shape of the asteroid using the ADAM, MPCD, and SAGE algorithms. The origin
of the shape was investigated by means of N-body simulations. Cybele has a
volume-equivalent diameter of 263+/-3km and a bulk density of
1.55+/-0.19g.cm-3. Notably, its shape and rotation state are closely compatible
with those of a Maclaurin equilibrium figure. The lack of a collisional family
associated with Cybele and the higher bulk density of that body with respect to
other large P-type asteroids suggest that it never experienced any large
disruptive impact followed by rapid re-accumulation. This would imply that its
present-day shape represents the original one. However, numerical integration
of the long-term dynamical evolution of a hypothetical family shows that it is
dispersed by gravitational perturbations and chaotic diffusion over Gyrs of
evolution. The very close match between Cybele and an equilibrium figure opens
up the possibility that D>260km small bodies from the outer Solar System all
formed at equilibrium. However, we cannot rule out an old impact as the origin
of the equilibrium shape. Cybele itself is found to be dynamically unstable,
implying that it was recently (<1Ga) placed on its current orbit either through
slow diffusion from a relatively stable orbit in the Cybele region or, less
likely, from an unstable, JFC orbit in the planet-crossing region.
