  The recent reported 750 GeV diphoton excess at the 13 TeV LHC is explained in
the framework of effective field theory assuming the diphoton resonance is a
scalar (pseudoscalar) particle. It is found that the large production rate and
the broad width of this resonance are hard to simultaneously explain if only
visible final states are considered. Therefore an invisible decay channel to
dark matter (DM) is strongly favored by the diphoton excess with a broad width,
given a large coupling of the new scalar to DM. We set constraints on the
parameter space in this scenario using the results from LHC Run 1, DM relic
density, and DM direct and indirect detection experiments. We find that the DM
searches can exclude a large portion of the parameter regions accounting for
the diphoton excess with a broad width.
