  The total mass density of the Universe appears to be dominated by dark
matter. However, beyond its gravitational interactions at the galactic scale,
little is known about its nature. Extensions of the quantum electrodynamics
Lagrangian with dark-matter coupling terms may result in changes to Standard
Model parameters. Recently, it was proposed that a network of atomic clocks
could be used to search for transient signals of a hypothetical dark matter in
the form of stable topological defects. The clocks become desynchronized when a
dark-matter object sweeps through the network. This pioneering approach, which
is applicable only for distant clocks, is limited by the quality of the fibre
links. Here, we present an alternative experimental approach that is applicable
to both closely spaced and distant optical atomic clocks and benefits from
their individual susceptibilities to dark matter, hence not requiring fibre
links. We explore a new dimension of astrophysical observations by constraining
the strength of atomic coupling to the hypothetical dark-matter cosmic objects.
Our experimental constraint exceeds the previous limits; in fact, it not only
reaches the ultimate level expected to be achievable with a constellation of
GPS atomic clocks but also has a large potential for improvement.
