  Using the 32-m Medicina, 45-m Nobeyama, and 100-m Effelsberg telescopes we
found a statistically significant velocity offset Delta V = 27 +/- 3 m/s
(1sigma) between the inversion transition in NH3(1,1) and low-J rotational
transitions in N2H+(1-0) and HC3N(2-1) arising in cold and dense molecular
cores in the Milky Way. Systematic shifts of the line centers caused by
turbulent motions and velocity gradients, possible non-thermal hyperfine
structure populations, pressure and optical depth effects are shown to be lower
than or about 1 m/s and thus can be neglected in the total error budget. The
reproducibility of Delta V at the same facility (Effelsberg telescope) on a
year-to-year basis is found to be very good. Since the frequencies of the
inversion and rotational transitions have different sensitivities to variations
in mu = m_e/m_p, the revealed non-zero Delta V may imply that mu changes when
measured at high (terrestrial) and low (interstellar) matter densities as
predicted by chameleon-like scalar field models - candidates to the dark energy
carrier. Thus we are testing whether scalar field models have chameleon-type
interactions with ordinary matter. The measured velocity offset corresponds to
the ratio Delta mu/mu = (mu_space - mu_lab)/mu_lab of (26 +/- 3)x10^{-9}
(1sigma).
