At room temperature the thermal population of vibrational excited states is low, although not zero. the initial state is the ground state, and the photons that were scattered will have a lower energy resulting in a longer wavelength than the exciting photon. This shifted scatter is what is what’s observed in Raman spectroscopy.
A small fraction of the molecules is in vibrationally excited states. The Raman scattering from a vibrationally excited molecules will leave the molecule in the ground state. The photon that was scattered will appear with a higher energy. This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, at room temperature it has enough for vibrational frequencies less than about 1500 cm-1. The Stokes and anti-Stokes on a spectrum contain the same frequency information. The anti-Stokes spectrum can be used when the Stokes spectrum is not directly observable.