  Advanced LIGO-Virgo reported a short gravitational-wave signal (GW190521)
interpreted as a quasi-circular merger of black holes, one populating the
pair-instability supernova gap, forming a remnant black hole of $M_f\sim 142
M_\odot$ at a luminosity distance of $d_L \sim 5.3$ Gpc. With barely visible
pre-merger emission, however, GW190521 merits further investigation of the
pre-merger dynamics and even of the very nature of the colliding objects. We
show that GW190521 is consistent with numerically simulated signals from
head-on collisions of two (equal mass and spin) horizonless vector boson stars
(aka Proca stars), forming a final black hole with $M_f =
231^{+13}_{-17}\,M_\odot$, located at a distance of $d_L = 571^{+348}_{-181}$
Mpc. The favoured mass for the ultra-light vector boson constituent of the
Proca stars is $\mu_{\rm V}= 8.72^{+0.73}_{-0.82}\times10^{-13}$ eV. This
provides the first demonstration of close degeneracy between these two
theoretical models, for a real gravitational-wave event. Confirmation of the
Proca star interpretation, which we find statistically slightly preferred,
would provide the first evidence for a long sought dark matter particle.
