  We examine the phenomenology of a simplified model of fermionic dark matter
coupled to a light scalar mediator carrying lepton number 2. We find that the
mediator can be very light and still consistent with laboratory and
cosmological bounds. This model satisfies the thermal relic condition for
natural values of dimensionless coupling constants and admits a mediator in the
$10 - 100 ~\text{MeV}$ mass range favored by small scale structure
observations. As such, this model provides an excellent candidate for
self-interacting dark matter.
