  The installation of forward detectors in CMS and ATLAS turn the LHC to an
effective photon-photon collider. The elastic scattering of the beam-protons
via the emission of photons, which can be identified by tagging the intact
protons in the forward detectors, provides a powerful diagnostic of the central
production of new particles through photon-photon annihilation. In this letter
we study the central production of dark matter particles and the potential of
LHC to constrain the cross section of this process. By virtue of the crossing
symmetry, this limit can immediately be used to constrain the production of
monochromatic gamma-rays in dark matter annihilation, a smoking gun signal
under investigation in indirect dark matter searches. We show that with the
integrated luminosity $\mathcal{L}=30~{\rm fb}^{-1}$ in LHC at center-of-mass
energy $\sqrt{s}=$ 13 TeV, for dark matter masses $\sim (50-600)$ GeV, a
model-independent constraint on the cross section of dark matter annihilation
to monochromatic gamma-rays at the same order of magnitude as the current
Fermi-LAT and the future limits from CTA, can be obtained.
