The First Constraint on the Dark Matter Millicharge via Multimessenger Time-Delay Analysis of GRB GW170817A

We derive new constraint on millicharged dark matter from the multimessenger observation of GW170817. In the neutron star merger event GW170817, the first detection of a gamma-ray burst (GRB) delayed by approximately 1.7 seconds relative to the gravitational wave emission was observed. Utilizing this delay, we constrain the millicharge parameter of dark matter within the large-scale structure of the Universe. {For dark matter mass below 10⁻¹⁵ eV, the millicharge parameter $ε$ is constrained to be less than 10⁻¹⁴, representing the most stringent limits achieved to date.