  Light particles quadratically coupled to nucleons induce macroscopic forces
in matter. While a quantum effect always exists, an additional force occurs in
the presence of a finite density of the light particles. We compute and
classify such background-induced forces for particles of spin $0,\frac{1}{2},1$
in the framework of effective field theory. We show that, at short distance,
the background-induced forces exhibit a universal behavior that depends solely
on the moments of the phase space distribution function of the light particles.
We compute the forces in the case of thermal-like densities of dark particles
that may realistically occur in cosmology. All the background-induced forces
remain, unlike the quantum ones, exponentially unsuppressed at large distance,
implying that large scale fifth force experiments are highly sensitive to dark
relics. Moreover at zero mass the forces from dark bosons are generically
enhanced with respect to their quantum counterpart due to Bose-Einstein
distribution. Overall, we find that the resulting fifth force bounds can
compete with those from quantum forces. A thorough adaptation of the results
from the E\"ot-Wash experiment may produce powerful additional bounds.
