  ZAIGA is a proposed underground long-baseline atom interferometer (AI)
facility, aiming for experimental research on gravitation and related problems.
In this paper, we study the possibility of detecting the ultralight scalar dark
matter (DM) with ZAIGA. According to a popular scalar DM model, the DM field
contains a background oscillation term and a local exponential fluctuation
term. In order to calculate the proposed constraints on DM coupling parameters,
we need to first compute the DM signals in ZAIGA. For the case of two AIs
vertically separated by 300 meters, the DM-induced differential phase consists
of three contributions, coming from the DM-induced changes in atomic internal
energy levels, atomic masses and the gravitational acceleration. For the case
of two AIs horizontally separated by several kilometers, the signal comes from
the DM-induced changes in atomic internal energy levels. With the current and
future technical parameters of ZAIGA, we then obtain the proposed constraints
on five DM coupling parameters. It turns out that our proposed constraints
could be several orders of magnitude better than the ones set by the MICROSCOPE
space mission.
