  Dark matter exists in our Universe, but its nature remains mysterious. The
remarkable sensitivity of the Laser Interferometer Gravitational-Wave
Observatory (LIGO) may be able to solve this mystery. A good dark matter
candidate is the ultralight dark photon. Because of its interaction with
ordinary matter, it induces displacements on LIGO mirrors that can lead to an
observable signal. In a study that bridges gravitational wave science and
particle physics, we perform a direct dark matter search using data from LIGO's
first (O1) data run, as opposed to an indirect search for dark matter via its
production of gravitational waves. We demonstrate an achieved sensitivity on
squared coupling as $\sim 4 \times 10^{-45}$, in a $U(1)_{\rm B}$ dark photon
dark matter mass band around $m_{\rm A} \sim 4 \times 10^{-13}$eV.
Substantially improved search sensitivity is expected during the coming years
of continued data taking by LIGO and other gravitational wave detectors in a
growing global network.
