  We discuss cosmological constraints on a dark scalar particle mixing with the
Standard Model Higgs boson. We pay particular attention to the dark scalar
production process when the reheating temperature of the Universe is very low,
which allows us to give a conservative limit on the low-mass scalar particle.
We also study the effect of the self-interaction of the dark scalars and find
this has a significant impact on the cosmological constraints. We obtain the
most conservative cosmological constraint on the dark scalar, which is
complementary to accelerator experiments and astrophysical observations.
