  We examine the constraints on sub-GeV dark sector particles set by the
proto-neutron star cooling associated with the core-collapse supernova event
SN1987a. Considering explicitly a dark photon portal dark sector model, we
compute the relevant interaction rates of dark photon ($A'$) and dark fermion
($\chi$) with the Standard Model particles as well as their self-interaction
inside the dark sector. We find that even with a small dark sector fine
structure constant $\alpha_D\ll 1$, dark sector self-interactions can easily
lead to their own self-trapping. This effect strongly limits the energy
luminosity carried away by dark sector particles from the supernova core and
thus drastically affects the parameter space that can be constrained by
SN1987a. We consider specifically two mass ratios $m_{A'}=3m_\chi$ and
$3m_{A'}=m_\chi$ which represent scenarios where the decay of $A'$ to
$\chi\bar\chi$ is allowed or not. We show that SN1987a can only place bounds on
the dark sector when $\alpha_D\lesssim 10^{-15}$ ($10^{-7}$) for the former
(latter) for $m_\chi\lesssim 20$ MeV. Furthermore, this evades the supernova
bounds on the widely-examined dark photon parameter space completely if
$\alpha_D\lesssim 10^{-7}$ for the former, while lifts the bounds when
$\alpha_D\lesssim 10^{-7}$ if $m_\chi\lesssim 100$ MeV. Our findings thus imply
that the existing supernova bounds on light dark particles can be generally
evaded by a similar self-trapping mechanism. This also implies that
non-standard strongly self-interacting neutrino is not consistent with the
SN1987a observation. Same effects can also take place for other known stellar
bounds on dark sector particles.
