I thought LIGO is kind of crude, i.e. it's being used to measure in the scale of blackhole level gravity. The dark matter experiment is trying to measure the gravity of a photon? May be too optimistic?

Sounds like they're looking for dark photons interacting with the detector, and, thanks to its extreme sensitivity, able to place upper bounds on the strength of the interactions (since none were observed). This means dark photons have very very weak interactions with normal matter.... or that they don't exist. :)