  The observed dark matter abundance in the Universe can be fully accounted for
by a minimally coupled spectator scalar field that was light during inflation
and has sufficiently strong self-coupling. In this scenario, dark matter was
produced during inflation by amplification of quantum fluctuations of the
spectator field. The self-interaction of the field suppresses its fluctuations
on large scales, and therefore avoids isocurvature constraints. The scenario
does not require any fine-tuning of parameters. In the simplest case of a
single real scalar field, the mass of the dark matter particle would be in the
range $1~{\rm GeV}\lesssim m\lesssim 10^8~{\rm GeV}$, depending on the scale of
inflation, and the lower bound for the quartic self-coupling is $\lambda\gtrsim
0.45$.
