  The microlensing signal in the light curves of gravitationally lensed quasars
can shed light on the dark matter (DM) composition in their lensing galaxies.
Here, we investigate a sample of six lensed quasars from the most recent and
best COSMOGRAIL observations: HE~1104$-$1805, HE~0435$-$1223, RX~J1131$-$1231,
WFI~2033$-$4723, PG~1115$+$080, and J1206$+$4332, yielding a total of eight
microlensing light curves, when combining independent image pairs and typically
spanning ten years. We explore the microlensing signals to determine whether
the standard assumptions on the stellar populations are sufficient to account
for the amplitudes of the measured signals. We use the most detailed lens
models to date from the H0LiCOW/TDCOSMO collaboration to generate simulated
microlensing light curves. Finally, we propose a methodology based on the
Kolmogorov-Smirnov test to verify whether the observed microlensing amplitudes
in our data are compatible with the most standard scenario, whereby galaxies
are composed of stars as compact bodies and smoothly distributed DM. Given our
current sample, we show that the standard scenario cannot be rejected, in
contrast with previous results by Hawkins (2020a), claiming that a population
of stellar mass primordial black holes (PBHs) is necessary to explain the
amplitude of the microlensing signals in lensed quasar light curves. We further
estimate the number of microlensing light curves needed to distinguish between
the standard scenario with stellar microlensing and a scenario that describes
all the DM contained in galaxies in the form of compact objects such as PBHs,
with a mean mass of $0.2M_{\odot}$. We find that about 900 microlensing curves
from the Rubin Observatory will be sufficient to discriminate between the two
extreme scenarios at a 95\% confidence level.
