  The standard model of cosmology, the LCDM model, robustly predicts the
existence of a multitude of dark matter 'subhaloes' around galaxies like the
Milky Way. A wide variety of observations have been proposed to look for the
gravitational effects such subhaloes would induce in observable matter. Most of
these approaches pertain to the stellar or cool gaseous phases of matter. Here
we propose a new approach, which is to search for the perturbations that such
dark subhaloes would source in the warm/hot circumgalactic medium (CGM) around
normal galaxies. With a combination of analytic theory, carefully-controlled
high-resolution idealised simulations, and full cosmological hydrodynamical
simulations (the ARTEMIS simulations), we calculate the expected signal and how
it depends on important physical parameters (subhalo mass, CGM temperature, and
relative velocity). We find that dark subhaloes enhance both the local CGM
temperature and density and, therefore, also the pressure. For the pressure and
density, the fluctuations can vary in magnitude from tens of percent (for
subhaloes with M_sub=10^10 Msun) to a few percent (for subhaloes with
M_sub=10^8 Msun), although this depends strongly on the CGM temperature. The
subhaloes also induce fluctuations in the velocity field ranging in magnitude
from a few km/s up to 25 km/s. We propose that X-ray, Sunyaev-Zel'dovich
effect, radio dispersion measure, and quasar absorption line observations can
be used to measure these fluctuations and place constraints on the abundance
and distribution of dark subhaloes, thereby placing constraints on the nature
of dark matter.
